How to tell this girl I don't want to be her friend?

this girls best friend left our school and that girl who is in 2 classes with me thinks that I think of her as a best firend but I don’t for lots of reasons. I can’t stand to be around her cause shes so depersate to have a friend she will do anything 1st of all shes really big the biggest girl in my grade (not to be mean) , she smells HORRIBLE like pee and other mixed smells. she is a suck up to me every thing I do she does or says she does. like one time at lunch my REAL best friend and I were talking about boys and she comes running up me(not a very pretty sight) and sits right next to me of course I lost my appitite and I said to my REAL best friend about this guy and she buts in and says yeah plenty of guys like me but I just don’t like them back im thinking yeah right. not much people even like her. and antoher day she painted her finger nails black like mine and kept trying to show me them in class. I don’t even like to be in my fave class cause shes there. I found out her mom only lets her take a shower once a week and the worst part is she still wets the bed . she yesterday I found out she has lice!!! im afraid I have it. today she came to school with a bandana I tried to avoid her the whole day how can I tell her I can’t be her friend anymore??? I need help!!