Since attractive just refers to outer features, if you're interested only physically, that's fine. If you want a relationship, pick something that describes your person of choice better or encompasses more features.
"I think you're amazing, would you like to grab a coffee/dinner with me sometime?" would probably work.
If you choose to be this direct, make it clear you don't need an immediate response. The other party didn't have any time to prepare for this, so they may actually need to think about you in this way first of all.
This may be the hardest part, because you will not know if you'll get a reply (depends on maturity and courage). You could ask them again once, but then I'd drop it and just assume it's a no, otherwise you may come across as pushy.
The reason most people don't do this is, that they're scared to be turned down or ridiculed (teenagers are assholes). I think from 30 onwards it gets a lot clearer and easier.
Good luck!