Will I gain a lot of weight if I take steroids like prednisone; I'm really afraid of weight gain?

Depends. It depends on much steroid you take and for how long. Even high doses taken for short periods produce less weight gain than more modest doses taken over a long time. If you do gain weight, some of to is due to the steroid itself (not your fault) but some it can be due to increased hunger and eating (in your control).