First of all, get yourself “in shape” … meaning, do 25-35 minutes of cardio activity 3-5 times per week. It’s best to get a mix of activities for your whole body, for example, running, biking, plyometrics, swimming, circuit training (weights), swiss ball, and/or good 'ol calisthetics.
It takes about 4-5 weeks to get “in shape”.
Not sure what part of the world you’re in, but if you can get outdoors then the next thing after cardio would be throwing — as long as you get into it GRADUALLY. Starting with maybe 20-30 throws per day and slowly working up to maybe 65-75, mixing in long-tossing, after 4-5 weeks.
If you’re a hitter, then you will want to get as many swings as possible, both off pitching and off a tee.
Basically, you want to already be in peak condition and have your arm ready to throw at or near 100% when tryouts start.