There are lots of good recipes out there, in books or online. Beware of any recipe that tells you to melt chocolate in the microwave (shudder).
My two tips are:
1. Make sure the baking powder isn't past its expiration date - otherwise the muffins won't rise - they'll be flat and horrid.
2. Mix all the dry ingredients first, mix all the wet ingredients together separately, and then very gently fold together just until incorporated. Too much mixing will make muffins solid and dense instead of light and fluffy.
Good luck and enjoy your muffins!