hi shortie
it's totally understandable to be scared, but if you can put it in proportion it will help. afterall, your gran managed it, your mum did it and if you have a daughter, no doubt she'll do the same.
i didn't have a chance to get really nervous because my DS arrived three weeks early taking me by surprise, but i'm sure i would have been terrified otherwise.
labour is painful, but it was a lot better than i thought it would be (honest!). contractions come and go and for most of the time, they are very manageable - after mine had been going for a few hours i had friends round for breakfast and managed to keep a conversation going with the occasional pause!
you will be ASTOUNDED at what your body is capable of doing, both during and after the birth and as you are only 20, you've got a big advantage.
definitely do some antenatal classes because after i'd done mine i felt SO much calmer about the whole thing. as everyone has said here, if it gets too much, have the epidural! i managed my labout using a TENS machine and gas and air, but i told everyone i would have the epidural if i needed it.
good luck, you'll be fine. i was so happy when my son was born it makes me weep to think about it even now!