It's quite common to have a hard time getting the guy's penis into your vagina the first time you have sex. This is for a few reason.
First, if you've not had sex before, your muscles are not used to stretching to accomodate anything larger than a tampon.
Next, if you haven't had enough foreplay (getting excited), then you might not be producing enough lubricant - this can be sorted by buying water-based lubricant. Other types of lubricant are not safe to use with condoms.
Finally, there are a lot of nerves involved with losing your virginity. It's normal for the vagina to involuntarily (or even voluntarily!) clench against the penis - after all, you've probably heard for years that the first time is painful, and that can be scary.
The best reponse I can give you is to just take your time. Try to have sex when you really feel like it, not just when you feel you SHOULD. Make sure to spend plenty of time messing around before trying to have penetrative sex, and use lubricant if necessary. It also helps if the guy is very slow about entering the vagina - shoving his way in can only make things more uncomfortable.
Finally, if you have any emotional worries about having sex (fears of pregnancy, disease, wondering if you are ready) this will have a HUGE impact on your ability to relax and enjoy yourself.