If you're in Rookie (or any other) League, you cannot customize anything yet! Only when you reach the Majors will you be able to customize anything such as your team name and uniform. That said, head over to 'My Team' and tap on 'Change Uniform'. From there, select a base uniform and change its colors using 'Color Customization'. However, if you want to make it look like how it is in MLB, you can pay for the MLB design by going to 'My Team' > 'Change Uniform', then tapping on 'MLB Design'.