How to tell a girl that you want only to stay as friends?

If you cant tell her truths, then tell half truths.

Im a PROOOO at half truths. You want her to be only friends so, instead of saying

"I dont want to go out with you or anything, i just want to be friends"

Say

"I dont like the poon, sorry"

But on a more serious note,
Just try to be as honest as you can and just tell her that you arent looking for anything right now. She will more than likely respect your choice.