How to deal with mean coworkers, and why are they so mean?

i work at ihop, by the way. most, but NOT all of my coworkers (some are very nice, supportive, and are very hard workers) are very condescending and i catch them giving me the dirtiest looks when i'm not looking and just happen to turn around. it's strange because i'm very nice to each and every one of them and i haven't given them a reason to not like me so far. they like to slide negative comments and make me look like an idiot whenever they have the opportunity to do so in front of our manager, and they like to "tattle" on me whenever i seem to make a minor mistake. it seems very little positivity enters their minds when they think or hear of me and i don't like it. they never have anything good to say and i feel like standing up for myself. however, i don't want to take the risk of becoming vulnerable and give my detractors the satisfaction of knowing i am bothered my their actions. what should i do ? should i keep a smile on at all times and pretend everything's okay ? or should i stand up for myself in front of my manager to prevent my name from being spread in mud because i've noticed my manager has been treating me differently lately.also, why are my coworkers treating me this way ? is it because of my appearance ?