A few things to consider,
You might have a very large penis (girth wise)
She has not had any thing let alone a penis in her before
Some guys during foreplay use a finger, just one finger. This is hardly the size of a penis(I hope)
if you are doing this, move up to 2, then 3, this will prepare her more for the main event.
Edit: Try some good lube, not Vaseline, go to the drug store, I have heard that astro glide works well.
I assume you are use condoms, if so, be sure they are pre lube and use a lube that won't break down the condom, petroleum jelly or Vaseline will do this, so don't use it.