You should get them a rose. It's a nice way to make them feel great, and hopefully they will have got you something too.
It doesn't neccesarily mean you are going out. You could though..
Good luck!!!
What do you do when you get asked to be someone's valentine and you say yes?
I guess take them on a date or hang out with him/ her.
If you want someone to be your Valentines, you do exactly the same kinda things you would if you wanted them to ask you our or date you: Flirt, flirt and more flirt.
Laugh at their jokes, give them compliments, treat them special and show that you're into them
If they are into you as well, then they should take the hint and do something about it.
If they're seriously not getting it but you're sure they like you, then you could just ask them something like "so... Who's your Valentine this year?" and if they say no one yet, you give them a hint that they should ask you!
Maybe get them a card or a present. They are sure to get you something too. :)