Contests usually have many rules. Each contest that's linked here explains who can enter, what you need to turn in and when you need to turn things in. Also, they tell you about the prizes you could win! Some contests are open to very young kids and kids as old as 17! Don't worry-- they judge your work against other kids your age. (A 5 year old and a 15 year old won't be judged in the same group!)
If one of these contests sounds interesting to you, ask an adult or an older sibling to sit down and read all of the instructions with you. Then, make a plan together so that you can submit your awesome work on time!