Wow, somebody has no idea what they are talking about. Go to the asterix if you don't care about why, and only want to know what you can do.
First, a capacitor can be simply thought of as two plates, separated by a dielectric. The dielectric has two purposes, it prevents the potential from each plate from traveling, and it serves to keep plate distances relatively constant.
With this simple model, the voltage rating is how much potential can be applied, before the charged plates will discharge through the dielectric. The Farad (what in Hades a Farah is eludes me) rating is how much charge can be developed. For the beginner again, one Farad is one Coulomb with one volt potential difference. The distance the plates are from one another determines how many Farads a capacitor can develop at a given voltage.
Now, the plate capacitor isn't reality. You've got film capacitors (very close to the plate, but rolled up), ceramic, metallic, and super capacitors. The later two types generally have very specific usage, so you're unlikely to see them. Unfortunately, the first two are insanely common. The next step boils down to capacitance and cost. Ceramic capacitors take a good deal more money to produce, so generally are limited to smaller voltages and capacitances. Film capacitors are easier to make when larger, so find their primary use in through-hole applications and where large capacitance values or voltages are in use.
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Bringing it all back around, make sure to choose the appropriate capacitor type first. You should be able to tell the type visually, as film capacitors will be large canisters while ceramic capacitors are oddly shaped.
Next, choose a capacitor with the right capacitance values. This value has to be accurate, as too much or two little will fundamentally change how the circuit works.
Finally, choose the voltage. You ideally want to be as close as possible here (to the original values), but choosing one with a slightly higher voltage rating won't hurt too much. The concern is that if you choose one too much higher the increase in plate spacing of dielectric will force the capacitance lower. Thinking about it as a hyperbole, the ISS and any metal object it orbits over are technically a capacitor. The spacing is so huge that the capacitance value approaches 0, and the potential needed to produce this capacitance is gigantic (think lightning on crack levels of insanely powerful).
