I'm 24 and have feelings for an 18 year-old
Do you think you have genuine feelings for her or is it just lust ?
I just want to stick my dick in her.
There's nothing wrong with being attracted to her since she is of legal age, but be careful not to view her as just a sexual object. Women are people too and deserve to be treated with respect instead of just as ways to get sexual gratification. If you are both okay with it though, then having sex with her is fine.
I'd love to do that.
You're free to have an consensual sex with any adult so I don't see any problem here but make sure not to hurt her by using her just for sex.