In the career menu, you can go to "My player" and then "Player equipment". You should be able to customize everything from hats to pads, bat included.
But if you've just started the career mode, I think it takes 2-3 matches before you can get to the Career menu. After each of the first 2-3 matches, you will be forced to go to nets to practice, hit the gym for a routine etc., after that you directly get put into the next match's starting screen, where you can only play the match or exit career mode.
After the first 2-3 matches are completed and these forced trainings are done, then you will end up on the career menu and only after that you can customize these cosmetic stuffs.
Other option would be to create and customize the player when the career starts, this would mean you'll have the customized stuff for the first 2-3 matches too.