14 year old girl here:
Call her up just for a normal conversation. Set yourself a minimum time to be on the phone to her(i.e 10mins). Prepare a list of things to talk to her about if you're that nervous. For example, what happened at school, what she's doing on the weekend, what you two are going to do together. Talk about rumours that are going round school. She will be really complimented that you took the time to call her and call her nearly every other day after that, slowly increasing the time you talk. Don't call her everyday(it's good if she calls you 2 but dont be put off if she doesnt!!) but call her at least every other day.
Don't worry if there are awkward silences, it's really hard to have a conversation with your gf/bf where there is non stop talking when you're young.
This way, you'll find it easier to talk face to face and as you learn more about each other, you'll discover if you're right for each other.
Hope this helps and let me know if it works! Me and my bf were in the same situation and he did this and now, we're really close and have been going out for 6months which is a record! OH, and if you can, say 'i love you' at the end of your conversation on the fone.