Hey bro,
that's all totally normal.
One thing that could help is to give her an orgasm first (from oral), this will help to loosen up the vaginal muscles initially + she'll love it and you should too :)
Whilst you go down on her, use her wetness to try and slip just one finger insider her and keep it there whilst she orgasms... this will slowly let her pussy relax and expand.
Then, when you want to have sex, try having her on her back, and you kneeling between her legs, then just gently ease into her. Maybe lean forward and kiss her and think about other things.
If she's nervous, or excited as well... she may accidentally be squeezing her PC muscles as well. Just take your time and enjoy everything.
If it's still difficult, go down on her again, and repeat :)
Things will get easier, just make sure that she is relaxed and breathing deeply as well.
Have fun and don't stress.
And remember... porn is NOT real life!!
:D