As your body matures, your cervical fluid (the discharge) changes throughout the month. Some days it will be yellow and sticky, some days white, some days clear like egg whites (btw when you are older, egg-white cervical fluid is when you are ovulating and most likely to get pregnant).
The brown discharge isn't exactly a period, but it could be signaling that your period will start soon. You should wear a maxipad with wings (get the ultrathin kind for comfort) so you are prepared for when it comes so you don't get your undies dirty.
Your "real" period will be bright red blood, and it will vary from heavy to light over the course of several days or longer.
As for masturbating, that won't bring on a period, and it's actually the safest sex you'll ever have, so enjoy.
I'm a senior year nursing student.