Ask a sibling or friend who is also trying out to practice one-on-one drills with you. Coaches will want to see that you can defend and attack no matter your position, and one-on-one drills boost your skills in both areas. Remember that though you may want to be a forward, if the coach has enough forwards for that year, he or she may want to see if you can fill other positions, too.
Set up this simple drill in a park or your backyard: use a cone or marker (a tree works, too) to represent two goals, and start against your opponent in the middle. Whoever dribbles to their "goal" first (you must dribble past it, not shoot from afar) wins that round. Play to 10 before stopping for a water break.