Investopedia is good, and there are a great many good sites online. There are also some very good books.
Anyplace where money is involved, there will be people peddling BS and hoping to profit from it- including selling poor advice to amateur investors. Study carefully, and if possible, find a mentor. A mentor would be an experienced investor you know, preferably a family member or close relative you can trust, and ask them to coach you as you go along.
Not all people should invest. The truth is that you need a certain kind of personality to be a good investor, and you need to be a person who is not ruled by emotions. There are many brilliant people who are lousy investors. My brother is one of them, and I manage his stocks for him with great success.
My friend and I (were both 16) run an investment LLC (with a parent as principle partner as per law), investment books are always a good place to start.
As the others stated, investopedia is a great source, wikipedia is also a good source to look up individual terms if you don't understand them. Learning about stocks takes time, be prepared to countless hours if you wish to really know the material, at the very least a few hours to have a basic understanding.
If you are under 18, you will need someone over 18 (preferably a family member you trust) to cosign a broker account in order to buy/sell stocks.
Investopedia.com
Resources for teens to learn about stocks?
