As a current White Belt I'll tell you now, knowing the techniques through visual studying and watching MMA since 1996, I am a very active athletic guy, played football, boxing, soccer, swimming etc... and then having just started rolling last December, it's worlds apart man.
If you wanna jump into a tournament, absolutely, I hope you love it, I hope it gets you addicted to the sport of Jiu-Jitsu but don't go in thinking you're gonna steam roll anybody. I hope you have fun and a good time but I also hope your Ego gets checked and you realize you need to start actual training at an academy.