The techniques you need to know.

Playing rhythm guitar leaves the player with literally hundreds of directions to go in. There's an infinite array of different rhythms and strumming patterns out there but how do you get that desired sound? The answer involves using a few different techniques at once.

Strumming: Strumming is the foundation to becoming a great rhythm player but it also tends to hold players up and limit creativity. Don't let this be a daunting task for you! Pick up your guitar right now and lets dive into this step by step. Points to remember before starting:

When looking at strumming patterns, the notation directing you to strum up looks like "^" and the notation directing you to strum down looks like an everyday table.
If you are playing an acoustic, remember to strum directly over the sound hole. Strumming in different places will give you different sounds but we'll cover that later on. For now, keep it simple and stick to one location and this will give you the best projection. The same applies to electric players at this point.
Take note of what your strumming hand is doing. Make sure you are using your wrists to strum and not your entire arm. This conserves a lot of energy and helps you do more intricate things. Keep your wrists relatively firm.
Strumming really, really hard isn't going to do much for your tone or ears. Try to keep it at a medium level. You don't want to sound too wimpy but sounding to loud can lead to undesirable buzzing sounds. Striking a good medium will give you a nice starting point.
So now take a basic chord progression, lets say G, D, and A minor. If you don't know these chords, click here to look them up. Strum each chord four times starting on an upstroke and ending on a down stroke. So the pattern for each chord would be up, down, up, down.