Help with starting a convo

I'm gonna start off saying that I am in 11th grade, I am a pretty confident person and i'm always smiling. I have no problem walking up to any girl and introducing myself to her. The problem is i get stuck on what to do or say next. For example, I see this really hot girl who we are going to call Jessica. I notice that she is sitting by herself at the end of gym class, so i walk up and move her stuff so i can sit down next to her. She already knows my name so when I sat down I just said "hey" with a smile on my face (not creepy, more like relaxed). She starts to smile and says hey back. All is good so far until I realize that I don't have a single clue on what to say next. It gets kind of quiet and i'm starting to get desperate for things to say to keep the convo going. I realize that i have failed so i say "well this turned out alot better in my head", she giggles and i leave. I'm planning on approaching her again tomorrow hoping that things will go a little better than before but I'm going to need some advice on what I should do or say once I get past the introducing part of the approach.

Say something about the dumb ass activities your gym class makes you do like ****ing kickball. Say how childish the activities are. Or how awful the lunch is. Just seem more mature funny and inteligent all at the same time. Just make sure your not all sweaty when your doing it. (Paper towels from the bath room. Spray of axe or whatever you use never hurt) just think about a subject (doesnt have to be a school subject) and go from there .

Except of saying just hi, you can say what is on your mind, be spontaneous. Sometimes that ends up great. You can try to joke or comment something around you.

all you got do is approach her as confident as you say u are and say hi with that same smile and say "i notice your here by your self want some compan? and work on from there....oh and am in the 11th grade too..good luck!