I met a girl online (pathetic, but not the point) and we talked a lot. It's been 3 months since we first spoke, but we haven't met yet. I like her a lot and don't know how to tell her. Wouldn't it be creepy without even having seen each other?
First of all, there's nothing pathetic about meeting a girl online — we spend so much of our time on the internet nowadays, and so many couples do meet online, that there's no longer much stigma attached to it.
Telling Her Online
Before you tell her how you feel, I think it's important to assess how much she likes you. Do you think she might feel the same way?
If you tell this girl that you like her, and she isn't interested in that way, then she might not be so keen on meeting up as friends. If you're willing to take that risk, let her know how you feel.
Tell Her In Person
Alternatively, you might want to suggest meeting up, see how things go when you're face-to-face. If she seems interested, then tell her in person.
This gives you the opportunity to back out—just in case she doesn't seem quite the same in person!
Ultimately, when (or if) to tell her how you feel depends on what you think is best, and how long you'll have to wait before you meet.
If you live close to each other, then I'd tell her in person. If you're hundreds of miles apart, tell her online or on the phone—and good luck!