This is the strum behind thousands of folk and pop songs: down, down-up, up-down-up. Count eighth notes, "1 & 2 & 3 & 4 &", and strum on 1, 2, & of 2, & of 3, 4, & of 4.
The key is G major, with open G, C and D. Downstrokes hit all the strings of the chord; upstrokes only catch the top three or four.
Keep the arm moving
Your strumming arm moves down on every number and up on every "&", like a pendulum, all the time. On beat 3 and the "&" of 1 it simply misses the strings. That's what keeps the rhythm locked: you never decide when to move, only when to touch the strings.
The most common mistake is stopping the arm on the missed strokes, which makes the pattern lurch. Play the pattern on muted strings first, saying "down, down-up, up-down-up" out loud.
Hold the pick loosely so it glides across the strings; aim for an even, bright sound with the downbeats a little stronger.
