The simplest thing to do is let someone else do the thinking. Go find a couch-to-5k program that you can follow for the next couple of months until practice starts. Races in middle school/junior high are usually 2 miles, so working from a beginner 5k program (that's 3.1 miles) should prepare you for practice with the team. You will certainly run more than a mile per day once the team starts meeting to practice together.
Cross country teams don't typically make cuts, so you'll get to be on the team regardless - tryouts are to determine who gets to run varsity (typically top 7), but for most meets the rest of the team will get to go and run as junior varsity.
As for your cramps, they are probably side stitches, but the other (thorough) answer and I differ on how to deal with these. I am never an advocate of trying to pattern your breathing. Your body knows how much oxygen it needs, it will determine how fast you have to breathe. Instead, make sure your breaths are deep and full, and so are your exhales. Train at appropriate paces for your conditioning level - side stitches often occur because a runner tries to sustain a pace that is way beyond what they are ready for. You overreach a little to get better, overreaching a lot just leads to pain.