see if you can find a little book called
Acoustic Rock for Guitar
by M. Wolfsohn
It's only about 20 pages (if that) but it has a good, basic rundown of picking and strumming styles as related to some well known players.
beyond that I'm goning to second Kirk's vote about listening to your favorite music and strumming along. Go ahead and mute all the strings with youyr
left hand and let that right arm find the rhythm naturally.
Good luck!
Ruben