If you're interested in the theory behind what timvass mentioned above, this tutorial explains some basic theory including how to create chords from the major scale.
Music Theory Basics
Something else to work on is try finding tabs to songs you like that aren't to complicated and work on learning how to play them. This is very good practice for chord switching and strumming/rhythm as well. Plus it is a lot of fun, especially at the beginning stages, when you can play a song that people can recognize.
Strumming/rhythm is also another fundamental aspect to work on. If/when you are pretty comfortable with strumming I highly recommend getting a metronome to help improve your timing as having good timing is extremely important. The metronome is very unforgiving and will reveal any timing issues you may need to work on.
-tkr