how do i make out with a girl who is high on weed?!

Oy! When a girl is buzzed all you need be is present and prepared, she'll do the rest if you're within arms reach. If you ain't there, the closest tree will do, if the stuff is strong enough. Embarrassing but true.

But you sound a wee young, so bring your older brother for backup, in case you wet your powder or lose your flint. Nothin' worse than being rev'd up with no place to go and being forced to do nothin' but hear a deflated boyo singing his sad sorry song. Be a dear and keep a man on deck.

It goes without sayin' condoms are first order of business, but methinks you're clueless enough to need it stated.

Just toke and let the weed to the work. It's easy.

Just do it.
Get some alcohol if she is way out of your league.

Same way you make out with a sober girl.