Take things slowly - you may find it easiest if the woman goes on top as that way she can take things at her own pace. Let the woman's body open up to the penis at its own pace - forget fast, deep thrusting, at least to start with. By going slowly and gently, you'll reduce the chances of discomfort or bleeding.
Lubricant can also help, though if the woman's vulva is entirely dry it may be a sign that she's not ready for sex. It's OK to say no at any point - even once you've started having sex - so if you realise you're not ready, don't be scared to tell your partner. Your first time is likely to be something you remember forever. Mutual trust, affection and respect will help make losing your virginity the happiest memory possible.