I really want to have sex with her ... what should I do?

Your girlfriend is being VERY sensible, whether you may happen to believe it or not. People with vaginas have a LOT at stake when they become sexually active⁠ (sexually active: Someone who has had or is currently having some kind of genital sex with a partner or partners.) . They are more prone to infections than people with penises. And yes, they can get pregnant, which is a very taxing process for her body, everybody's nerves and the bank accounts of those involved. Unwanted pregnancy⁠ (pregnancy: The state of carrying a developing embryo or fetus within the uterus. Medically, someone is considered to be pregnant when an egg has been fertilized by sperm, cells divide, and the fertilized egg is implanted within the lining of the uterus.) is obviously a concern of hers.

She is right to say that it is not a good idea to have sex⁠ (sex: Different things people choose to do to actively express or enact sexuality and sexual feelings; often this involves genitals, but not always. ) until you are prepared to deal with having a baby. Being prepared to have a baby is not the same thing as deliberately making baby every time you have sex (i.e., the idea that sex is only for procreative purposes). She just wants to make sure you both have a plan in place in case something goes wrong.

Here's what you do:

1. Don't pressure her. If she says she's not ready, pushing her into having sex is going to make her feel awful about herself, about sex, and about YOU.
2. Leave her an open door to come around when she is ready to have sex. Till then, you should discuss your plans for things like birth control⁠ (birth control: Any number of methods people use to intentionally prevent unwanted pregnancy, including the condom, the cervical barrier, the implant, the patch, the pill, the rhythm method, the ring, the shot, the IUD, spermicide and withdrawal.) , doctors visits, STI⁠ (STI: Sexually transmitted infections: illness, infection and/or disease which is often or can be transmitted through sexual or other intimate contact, like HIV, Chlamydia or Herpes. Some people call STIs STDs.) testing, condoms, and how you would want to deal with an unwanted pregnancy. You must accept that there is a possibility that she may never be ready to have sex with you. She's only 14, and a lot of girls don't start having sex until many years later.
3. Read Ready or Not? The Scarleteen Sex Readiness Checklist, and think about whether you really have the capacity to conduct a responsible and healthy sexual⁠ (sexual: About or relating in some way to sex or sexuality.) relationship⁠ (relationship: An ongoing interaction or association with another person, place or thing.) . If you're sure you can hold up your end of the partnership in this respect, then read the checklist with her.
4. Also read Be a Blabbermouth! The Whats, Whys and Hows of Talking About Sex With a Partner and Sexual Negotiation for the Long Haul. These two articles will give you more help with your problem.