What can I do that would be nice for my girl? To let her know I love her? I am 18 and my girlfriend is 17 I want to to do something really special to let her no I love her and I want to be with her for ever

First, it has to be age appropriate. So that we can give you good ideas it might be good if we know your ages.

Buy her a neckless or something like flowers

A nice dinner at a decent place is always nice. Appeal to her intellect, not your hormones.