I need advice about a girl I like?

I like this girl, let's call her aly, now aly was a new girl 2 years ago and I had her in my language class now I really liked her and this other guy in my French class who I thought was cool ended up telling her I liked he and something else because whenever I tried talking to her it was am awkward silence so now, I still like her and after a while we had another class together (I'm a sophomore and so I aly) she went around telling people I liked and all this crap but I denied it and called her out and now I have another class with her it just started mechanical drawing, now I'm a pretty big guy (250) and she's a petite thing, I'm going to try and sit by her tomorrow but anytips how I get this thing going to maybe ask her out? Be serious anything works thanks :)