Playing reggae on guitar, particularly if you're not overly familiar with it, can seem very daunting. But like just about anything, the more you play around with it, the more it becomes second nature. I recommend starting out with a very basic beat to start out with. It doesn't even have to be a reggae song, but it should be something you know well and that does not involve a lot of chord changes. The classic “Stand By Me” works well as do many simple songs.
Two things to remember: be sparse and clean. Try starting out with this simple rhythm pattern:
1 + 2 + 3 + 4 up up up
You'll see that you only use upstrokes. Until you get very comfortable with reggae, I find that this technique helps to really concentrate on the thythm - and it sounds very good, as well. Go lightly on the strings. If you're using a Strat-style electric guitar, then I would also advise you to use one of the “out of phase” settings (second or fourth position on a five position switch). Keep it relatively clean - more high end than bottom and just a touch of reverb or chorus.