Relays are basically switches. There are two main leads, which, when voltage passes through, switches the switch. Then there are usually about 12 other leads, each a separate switch, whith the main hot, hot when on and hot when off, for each. This may vary, as the relay may only act as two switches, and therefore have the two main and six other leads for the two switches. It is actually quite simple to figure out without a wiring diagram. Just use the 'continuaty' mode on your tester, and that way you can figure out which lead is hot when there is not voltage applied to the main leads, and then apply voltage to the main leads (for most relays 12v is fine), and test to see which leads are hot when voltage is applied.
Relays most likely have a max voltage rating, but the more important rating is the maximum amps rating. (Actually, there will probably be two voltage ratings, one will tell you how many volts to use to activate the relay, and the other will be how many volts the relay can switch). So basically, what you need to be concerned about it how many volts you use to activate the relay. I used an old automotive relay, rated for 12v to activate it, but I had an old 6v power adapter lying around, so I used that instead, and it worked fine. THe other thing is how many amps the relay will need to switch. I presume that you will the LED circuit that you will be switching will be no more than 12v, so voltage will not be a problem. What you need to figure out is how many amps will the circuit be running at. Once you have figured this out, you must make sure that the relay is rated at a higher amps than that. The reason for this (of which I can bear testiment) is that if you overload the relay (pass too many amps through it) the relay contacts will get so hot that they will weld together, rendering the relay useless.
Keep on having fun,
Cheers,
Stephen.