Remove the battery cables and clean the outside of the posts and the inside of the cable clamp.
you can buy the post and cable cleaning tools from walmart or other places for under $5.
Remove the negative (black) cable first, then the positive (red +). Re-attach the cables - positive first and negative last. This order will help prevent sparks and other problems.
If your battery has removable cell caps, when you get the post cleaning tool, pick up a battery specific gravity tester and check the charge in each of the battery cells. That will give you an idea of the state of charge.
You can read up on battery service here.
http://www.uuhome.de/william.darden/carfaq.htm
If the cleaning doesn't help, then get the battery charged and load tested at a shop.