Condoms can be used for prevention of pregnancy as well as reducing your risk for sexually transmitted disease and HIV. You do not mention if your girlfriend is a virgin as well.
For prevention of pregnancy put the condom on before you insert your penis into her vagina. ANYTIME your penis is inside her vagina, you need to have a condom on because 1. Men secrete a small amount of "pre-come" prior to ejaculation that contains a few million sperm, and men cannot always tell when this secretion takes place and 2. Some men do not have very good ejaculatory control and therefore come inside their partner when they don't want to or mean to (oops!). Having a condom on will help with that. If you are looking to use a condom for protection from disease as well, then you should follow the above instructions and ALWAYS have a condom on when in or near her vagina. You may also want to use a condom if you engage in oral or anal sex, as it is possible to contact a sexually transmitted disease through these sexual acts as well. Do you know how to put on a condom correctly? You can practice by yourself if you are comfortable, and then when the time comes you will be more at ease with using a condom. Put the condom over the head of your penis and ROLL IT DOWN all the way to the base of your penis. Make sure you leave a little room at the tip to collect the semen. DO NOT unroll it and put it on like a sock!! When the condom is on all the way it is OK to have sex. You can check the condom during sex to make sure it has not slipped off or broken. When in doubt replace it with a new one.
After you ejaculate, grab the base of your penis where the end of the condom is and withdraw from her, being careful not to let the condom slip or spill inside her.
Dispose of used condoms by wrapping in tissue or toilet paper and throwing in the trashcan. DO NOT flush then down the toilet!! You will clog up the septic system!
As for a step between foreplay and intercourse...The first thing I recommend is TALKING to her and asking her what she likes. Good communication is the key to a healthy sex life.
Take your time and explore each other's bodies. It may take a little while to find your groove and find what is good for both of you. Keep trying! Sex can only get better as you get to know each other better and are willing to work at it.