In the old days it used to be a little easier as there were fewer types of crank/bottom brackets to choose from but now it can depend on what style you have and this governs the tools you might need.
Unless you have the now more common external bearing bottom brackets like this:
http://www.nashbar.com/bikes/Product_10053_10052_502313_-1_202630_10000_202430
where the cranks can be changed with an allen wrench, you’ll need some special tools I’m afraid. I don’t know how much you are willing to spend and how keen you are to keep fixing your bike over the coming years but it may not be worth it to buy the crank extractor tool that you’ll need.
This would be a good time to get acquainted with your Local Bike Shop who can fix this for you.
Just a warning: New cranks means new chainrings (cogs on the cranks), which might not work with an already worn chain. A new chain and also new rear cassette (cogs on back wheel) might be needed too. Again another good reason to take it to the bike shop.
Good luck.
Paul G
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