There are many ways to muffle the strings that you don't want to ring. You simply have to get in the habit of fingering your chords so that some of your fingers lightly cover the strings your aren't playing. You will soon get to where you don't even have to think about this at all.

To better help you; can I ask which chords you are playing? I can give you a step by step breakdown of how to approach each chord and where to place your fingers.

Please specify when you get a chance...... :o