The advice we give you would probably be more efficient if we know if you're confessing to a girl online or in person.
Lots of people seem to think you should never tell a girl you like her, but I don't think you should think that can't work, if that's what you want to do. Personally it's just not my style. In my most recent close-distance relationship, I just asked her if she wanted to be my girlfriend and she said yes. Actually, my words were, "Would you like to be...intertwined?" It was pitiful! But that should show you that it doesn't matter how you go about doing it if the girls wants to date you. If she wants to, she wants to. If she doesn't, she doesn't. The way you go about asking probably won't make or break the deal, based on my experience at least. The most important thing is to get to know her and see if you two connect well.
If it's a long distance relationship over the internet, you really don't have the option of "implying it" like >>3 suggests. You pretty much have to say you like her. You can't even ask her if she wants to "go out" since that obviously can't happen! I think timing is more important in this situation, and in some ways it can be harder. Your best bet is probably to casually bring up something related to dating and hope for an opening to throw in your confession.