Handling gossip and rumors can be incredibly difficult and it can be really hard to understand why anyone would want to start a nasty rumor about you. It's important to keep in mind that if someone has started a rumor about you that is just completely false, it's honestly just to do with their own insecurities. They're just trying to hurt you to make themselves feel better.
So try not to take it personally. What you want to do is make sure that your best friends know the truth. When someone wants to know something they always go to the best friend, right? So make sure that your best friends are able to say "Oh no, of course that's not true." And they can help you set the record straight. If you hear someone in the hallway talking about this nasty rumor going around, don't confront them and get in their face and be like "That's not true. Why did you say that?" or anything super dramatic and ridiculous and something that belongs in a reality show. Instead, casually drop in the conversation later when you're all talking.
"Oh. I heard this ridiculous rumor about me today. Of course this isn't true." Play it off casually like you don't really care that that rumor has been started because that's not true and that kind of stuff doesn't bother you. Right? At least pretend like it doesn't. The best thing I can say about dealing with rumors and gossip is to relax a little bit and that's the hardest thing to do when you hear that other people are saying things about you. Just try not to worry about it. The truth is everyone is going to find something else to talk about in like 5 minutes. So if you can get through it you'll be very happy you did.