What type of capacitor should i buy?
Without specific examples (i.e. links to the circuits you found) it's hard to give an authoritative answer.

In general:


Capacitors specified with no polarity (no + and -) and no voltage rating are usually ceramic types, and common voltage ratings (25V, 50V) should be fine.
Capacitors that show polarity are polarized types - usually electrolytic, sometimes tantanlum. Substitution with ceramic or other types usually isn't possible as the capacitance needed may require many in parallel. Polarized capacitors in the tens of microfarads or higher are generally electrolytic. Lower values could be electrolytic or tanatalum.
A voltage rating is a helpful hint. Never place a cap with a lower voltage rating where a higher voltage rating is indicated.  A good guideline is to choose the capacitor with voltage rating, which is twice the nominal voltage across the capacitor in the circuit.  High voltage, small values (picofarads to a few nanofarads) are generally ceramic; high voltage with higher values (tens of nanofarads and up) are often metal film or polypropylene capacitors.
Capacitors that say X, Y, X# (i.e. X2) or Y# (i.e. Y1) are safety-critical and must be special ceramic or film capacitors that bear safety markings (UL, TUV, etc.)