Have some lube on hand, and maybe some toys (of various sizes) if your fingers are not terribly long. (Your fingers are way preferred, though.) If you're having sex with her, you should be able to talk with her honestly and without criticism about her past experiences with fingering, dildos (or dildo substitutes), and the like. She may already have taken some sizable toys, though odds are low.
Maintaining eye contact during the foreplay and act is very important because you are connecting emotionally as much as physically. Your eyes (should) tell her that you take her seriously as a person, that you care about her -- and also that you are feeling it too and that this isn't an emotionless experience for you. Her eyes give you important feedback about what she is feeling, how much is too much, what is working and worth further exploring, and what is a bad idea at this particular point in the festivities.
Be aware that she may be very concerned about disappointing you and may de-emphasize her own pleasure (or attempt to hide her discomfort) so that you "get there". Be very sensitive to this and reassure her as necessary that you're most interested in a mutual level of pleasure.
Some louts on this thread have advised getting her drunk -- ha ha. But getting her "drunk" on orgasms is an excellent idea. Some finger and tongue foreplay, where she's responding in crests after a while, is a great way to lessen her nervousness and also get her suitably wet.
One last thing: this is an intimate act, physically and emotionally. It requires a lot of trust, especially for a first-timer about to try to handle a larger-than-average tool. Do not belie this trust. If you find yourself thinking something like, Hey, this is going well, I'll just try a little thrust here -- don't. You can easily make a painful (to her) mistake that can take a lot of relationship work to undo. Take your time. She will signal you when you can go faster and deeper.
Best,
Kev | 
