Here is one more way of playing a shuffle. This one involves mainly the 2nd, 3rd and 4th strings.
If you've got this far, your rhythm playing should be sounding very good. One of the best things about being a good rhythm guitarist is that lots of people will want to play with you, since most guitarists just want to play lead and often neglect developing their rhythm skills.
Here is a variation on the previous shuffle, this time in the key of A. The first finger is used to cover each of the first thre strings for much of this progression.