2015 player,
Most every player invited will be a 2013 or 2014. A 2015 would need to be as talentd as the very best 2014 and 2013s. Delmon Young and Justin Upton types, they both were invited at an early age.
First thing... You need to be identified by the scouting community. You will need to be recommended by a MLB scout even for the tryout. If you are unknown to the scouting community, I suggest you attend a few tryouts held by MLB Clubs. There are other events you can attend that will help scouts identify you. If you want to get a better fell for the process... think about attending. just watching, the tryout this year to see what it's all about.
Only two ways of getting identified. The scouts find you or you find them. Only one way that you have some control over. If you're in the Bay Area it shouldn't be that difficult. Lots of good baseball opportunities for talented players out there. It can be tougher if your located a large distance from there.
Best of luck