Watch the Finding Snare Sounds online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
Now the snare. We usually hear it on beats 2 and 4, the backbeat. Mute the strings with your left index finger, then snap a string.
I get that snap by grabbing the string underneath and pulling up then letting it snap back. Don't pull too hard, because you might pull the string right off your guitar! Just enough to get that snap sound but not too soft.
The D string is my favorite sound, but try the other strings too.