If the toilet is 'running' as often as you say, it's probably because the flush valve needs to be replaced. BUT, before you get too far, here's a link so you can educate yourself on How Toilets Work. Load up the page, and on the left side is a link for 'How Toilets Works'. How Stuff Works.
My best to ya and hope this helps.
PS: When you do the 'fix', make sure you make a mental picture of where the hook-ups are for your Flush Valve Chain. If you put it back in the 'wrong' setting, it's not the end of the world BUT you may be frustrated in figuring out where the proper setting is.