Watch the Day 2 online guitar lesson by Ravi from Learn Guitar in 21 Days
The most basic strum pattern is four down strums while counting "1-2-3-4." You'll find that even this most simple approach will give you the sound and feel for almost any song as long as you sing the melody and change the chords at the right time.
So, don't get too caught up in strums, as the best approach is to start any new song with this basic down strum. Then, hit the strings on the way up as well, the down-up strum while counting "1 and 2 and 3 and 4 and." This way you are making twice the amount of sound with the same hand motion, giving you a little more movement and fuller accompaniment to a vocal line (melody).
No matter what strums you play, always count out loud while strumming--it's the best way for you to learn to play in time.