What is SPOJ?
SPOJ – Sphere Online Judge – is a problemset archive, online judge and contest hosting service accepting solutions in many languages.
The first thing you need to do is to register (it's free and spamless). Then find problem from one of the extensive problemsets, write a solution in your favorite language and submit it. Next switch to status page and watch progress of judging of your solution. After few moments you will see the result:
AC - accepted - your program ran successfully and gave
a correct answer (this you will like most :) )
2. WA - wrong answer - your program ran successfully, but gave an incorrect answer
3. TLE - time limit exceeded - your program was compiled successfully, but it didn't stop before time limit
Simply solve more problems :)
To get more familiar with SPOJ read one of the availabe tutorials or find answer in FAQ.