Breaking in a new vagina is always difficult, unless the girl did the work for you with a dildo or something. Pain, tension, fear, etc will go away with time if you play your cards right.
You might want to ask her this first. Can she use a tampon? If no (I couldn't for a long time) then she's definately in need of some help with her hymen. My first broke it just by being rough but I don't recommend that.
If she can use a tampon, then a finger should fit just fine. So after that it's a matter of using one finger, then two, then three, and then your penis should fit too (like Slauncha Man advises). But this whole procedure can take a VERY LONG TIME. That is, over an hour. Possibly multiple sessions. It did for me, and when it did break, I bled enough to be scared. Good news is, it doesn't hurt
So in short, the main thing to keep in mind is, everything takes time, especially vaginas!