Hi Harry! How's it going?
I get this question all the time whenever I hop down in the beginner's matches to help out. The problem is the beginner's matches are for 1-3 level players only, and they kinda lock the ship choices and gun load outs to 6 very good solid choices, just so that completely new players don't shoot themselves in the foot by bringing a combination that is horribly underpowered and thus they might get discouraged with the game.
There is no way to pick other ships in the beginner's matches, but if you're ready to get into some of the more competitive matches, you want to select "No" for the "Novice" option at the bottom of the Match selection page. You'll see something that looks like this:
Change that to "No" and you're good to go!
That being said, make sure you study up as much as you can in the Novice matches. It's a very low-stress environment to learn all the basics, and when you think you're ready for higher competition you can venture forth into the great unknown. Just beware, there be dragons out there!