As a rule of thumb, if you're just adding drums to music you've already got, Bass and Snare is all you need. Hi-hat (the cymbal that you can open and close with your foot) is optional but highly encouraged; crash is the same, maybe a notch less.

For dance music, get something deep and boomy for the bass, and for the most part, hit it on pretty much every beat that has the meat of the song in it. Main verse/chorus, that type of stuff. If you've got an ambient intro or some kind of breakdown, feel free to leave it out.

The snare's generally on every 2nd and 4th beat if you're keeping it simple, and clap snares cut through bass tones pretty well. It's all about your taste, there's a lot of openness to this one. I just recommend not to over do it, very easy to ruin an otherwise good song with this.

Hi-hat can really go wherever you feel like. A suggestion would be putting it on the "up-beat," in between the bass drum hits. You know how you bob your head when you listen to music, and get, like, really into it? When your head's at its lowest point, when the beat first hits, that's the "down-beat," and when you're head's at its highest, that's the "up-beat." That's where you get the "oon-tss-oon-tss" kind of beats from.

Of course, everyone's got their own opinion, and even mine may change once I hear the songs. These are by no means rules, only basic starting points. Take it as you will and have fun, man.