You can become a mormon (or as we refer to it - being baptized a member of the Church of Jesus Christ of Latter-Day Saints) any time you want - at any age or time of the year. It is a very big and important decision. For someone your age - the church requires that anyone requesting to get baptized that they need permission from their parents/guardians. 
 
To start things off I would encourage you to contact the missionaries of our church. If that is not workable - then talk to someone at school who is mormon. They will be happy to talk to you about it. Or you could just start right here at www.mormon.org. 
 
Although you can become a mormon at some time in the future - people who do join almost always say that they wish they would have found it sooner or joined sooner than they did. But this is a personal decision that you will have to make for yourself. 
 
If you have further questions - don't be afraid to ask.