Here you will find over 1000 of problems. They are like the ones used during programming contests, and are available in HTML format. You can submit your sources in a variety of languages, trying to solve any of the problems available in our database.
First of all, read carefully Onlinejudge Coding Guildlines and Frequently Asked Questions.
Then, choose a problem, solve it, submit your solution, and discuss with others.
Any problem, just write us.