This is a long version of what you may be getting yourself into and I was just like you, wanted to charge nothing, so it would be nothing. You will find you are a valuable asset to real people because this is obviously some sort of passion of yours.
Please don't read if you hate life stories
I started tech-ing around Windows 3.1(1996ish), upgraded ram because i couldnt play Dragons Lair due to Insufficent Virtual Memory. Then i found out the hard drive was full. Then I found out ZIP Drives. Then I found GEOCITIES. Then I found .MP3s. Then I couldnt get off the damn computer. I tried school and have taken classes since Windows 2000 Professional. I have never completed my darn A+ or any other certification for that matter but have nearly 40 college credits. My "real jobs" before computers were serving/bartending. My life revolved around partying & restaurant girls, luckily the plural was a continuous "s". But I wanted change.
I found a job at Staples, selling office equipment and was there for the launch of EasyTech. I was named the lead and hired a couple guys I went to school with. We had no internet connection, we had no guidance, we were winging it but luckily we were very good at slip streaming SP2 for XP and then VISTA came. I ran away from that place SOOOOO fast and went back to bartending.
I then was talking computers with everyone I served and worked with. Thats when the PCs started being handed to me.
Computer repair as a hobby in the beginning is very rewarding because of all the horror stories and dirt-hats you hear that are out there. You show Grandmas and Grandpas keyboard shortcuts and they think you invented the wheel. You show your Uncle MSCONFIG/IPCONFIG, he thinks you have a degree. You upgrade ram for your buddy, he wants you to build a gaming rig. How FUN is THAT!
I started your very venture around 2006 and went "donation" based to keep a casual understanding. I never once had anyone sign anything and luckily never fubar'd a PC.
Boy did that get me more than i could handle! Before you know it I was entering several homes & businesses a week. I quickly adjusted my rates because like a wanted man, the reward is high. I went to $30/hour for on-site and had no problem getting that. If I took something home, I would charge flat and very reasonable rates because nearly every PC I worked on I saved their data and REinstalled Windows XP(thank Gates for TechNet). I occasionally ran into the "wheres my program" people but I usually was very clear up front of what the goal was. "I will make you feel like you PC was like it was day 1, without the bloat but that means no software" Nearly everybody told their friend, etc etc. I had 3 - 5 PCs/House Call to work on a day! I learned WAY TOO much about everything and nothing at the same time but was very happy doing these tasks and getting smiles. I still reported it as a hobby because I had no intentions of going LEGIT.
Walking downtown, I saw an office, only $300/ a month. WHAT! I though legit meant $1000's! I wanted to go legit!
About a year after I had started this side hobby, I had an office, clients and a windows/mac decal on the back of my truck that said "Hi, I Fix and Sell Computers." My office was nothing crazy but something to keep all my PC stuff away from home. I was still bartending nights and playing golf nearly everyday. I was on top of the world.
Then the PHONE CALLS. More business, meant more work, meant more follow up. Holy crap was I in over my head but kept at it and QUIT MY JOB.
About 2 months into the office, I had already pitched a 20+ office company contract (me and a friend(from staples) fixed their server during the interview but didn't get the contact. Just a nice thank you card.) I built a front page of a website ( I am terrible at design but tried and failed!) and I had "cracked" an ACCESS database for a company and converted it to Excel so they no longer had to pay monthly dues (easiest $2000 I ever made) All of these were over my head tasks but I took them on without hesitation building my versatility, knowledge of my limits and "balls". All of this was still while getting 1-5 PCs/House Calls a day.
Then I got a call, my friend got a call from a HUGE LA clothing company and asked if she knew any "hackers". She called me, my golf game felt offended, my computer skills felt under-qualified. However, she had some computer issue while I was on the phone, I told her to do 2 things and it worked. Needless to say the company called me the next day haha. I flew out, met execs and showed them all the weird things I knew about what I felt was nothing. They were blown away, partied for about a week and flew back home. They paid for everything! A day later, they call again and say they want me to manage a SEO project for a minimum of a month and what I would need to do that. I calculated my $30/hour plus expenses and said $400/day, they said you're flying out tomorrow. I had an apartment waiting for me and someone picked me everyday to go to the MASSIVE warehouse for a couple hours a day. I was again in over my head but went for it. Their IT team was over 50 people and they looked at me when I started describing what my SEO plan was and had no idea what the hell I was saying. Basically created a farm of side interests of the company/owner to push down the bad publicity. I was pinging the IPs of the sites we didnt want up in the searches. Jumping from hotspot to hotspot. Learned Cain and Able. Kept pumping content on all the domains I was creating and Somehow it worked haha. Thank google's shoddy algorithm(back then). This was the first 2 weeks, the next few are still a blur as I was in LA partying with these crazy people who thought I was a genius hacker. I was very un-confident about the actual work i had done and hadn't proved anything to myself. That month was over and I flew back home. They asked for an invoice and I had a $8000 check in a couple days. I was stunned! I was going to be 1099'd. Had no idea what that meant and kept going.
I then consulted a business accountant and she tried to whip me into shape. KEEP ALL RECORDS she said. (rigggght) I had to have insurance for my office(contents and liability), for on-site(in case anything happened to me or their equipment) and invoices/reciepts for all income and expense.
I didn't want to be legit anymore haha. I moved my office by the golf course and started doing more jobs. Then ended up working for the golf course as an employee and dropped the office. Then I moved to WA and had to take a job again at staples (uhhhhh boy have they changed for the worse, I was a tech but was pushed to stop working on a PC that was due in an hour to try to sell warranties all day) but then luckily landed a gig with a startup and relearned Google's algorithm and got deeper into SEO the last couple years.
But I wanted my independence, so I am back to doing my own thing and have a LLC set up
SOOOOOO....
If you want to run it as a hobby, you will report as OTHER INCOME on your 1040. You can still write off expenses and I would recommend you do so. Keep business card costs, flyer costs, hardware costs, etc. There is no determined amount that deems a hobby but that OTHER INCOME is taxable as the IRS will want their .02, even if its .02. Depending on the amount, you may get a letter from Social Security saying hey for your future and for us we need you to pay x % also.
Once you get rolling, you will want to take advantage of the deductions available to you. Mileage, home office, internet, cell phone etc.
You have options. Set up a Sole-Proprietorship or LLC
Sole-P you will have a larger tax liability and MUST pay into Social Security based on NET PROFIT(Income - Expenses) but its less upfront paperwork and you fill out a schedule C
LLC, you are now a Corporation and you can never personally be liable for issues. You pay yourself as an employee and the company is responsible for half your tax and ss liabilities. Get a good accountant.
I still didn't have anyone sign something. I have had a couple bumps but I am very casual and make it up to people 10 fold. I tried to solder a power jack ooops no go, here's your data and your laptop is toast, no charge.
I plugged a power supply in POP, I'll buy you a new power supply.
Honesty can only go so far though. Protect yourself somehow because there are sue happy people out there and I don't want to see you on Judge Judy with her getting mad at you because you arent a legitimate business.
Judging from what I read from you, I would say you are 20 - 24 years old. At that age the last thing I wanted to do was join the "government way" but 10 years later I say its for the best and will make you a better man. Plus that 1 year old will have a better life/future, if you do it right and that will give you all the satisfaction you could ever ask for.
Go at it as you wish and you may learn some hard ways but sometimes thats the best way.
Keep in mind the "hey quick question" phone calls that will start. Those frustrated me once they got to be 5+ a day. There are many things I skipped over but just enjoy your life and craft, good things will happen.
Best of luck.