First of all, Kern112, let me extend a great big warm welcome to Pixies'! I'm glad you got up the courage to post --- we'll do our best to help you out! I hope you'll continue to ask if there are other questions on your mind, and generally just jump into the forums and enjoy yourself!
Now, as for your question:
I think that Phoenix is right: if your g/f has only tried the missionary position, the next position she should try is the woman-on-top position --- it's just as natural, but it gives her a little more control over what's going on. And ooooooh does it feel good!
There have been a number of threads posted on this subject in the past, 2 of which spring immediately to mind. In both cases the questions were posed by women, and you may find it helpful to have your g/f read the answers herself (since the answers are definitely directed towards the female viewpoint)... There are lots of other threads if you dig back in the forum a little farther, but these ought to get you started...
Advice for my 1st time on top:
http://www.pixies-place.com/forums/...=&threadid=5963
New w/ question:
http://www.pixies-place.com/forums/...=&threadid=5821
There are other positions to try, of course --- doggie style is one popular position that she may like, since although the sense of being in control when on top is wonderful it can also be a little daunting if you don't know what to do, and doggie style would shift the 'task' to you... Of course, there's also one of my personal favourites --- oral sex, with your head planted between her thighs to bring her as much pleasure as humanly possible. (I looooove to give oral sex too, but that's another matter...

)
One thing to ask though is why she's never done anything other than missionary. If it's just because of inexperience, that's fine and she'll be thrilled to learn all these other variations on the act of lovemaking; but often it goes beyond inexperience to a sense that sex is somehow wrong, particularly outside marriage. If she has had that kind of introduction to sex, as so many of us have (through religion, family, and sex ed at school), then chances are that the missionary position seems to her the least 'dirty' of all the positions available, and so it's easiest for her to get over her inhibitions about sex in general in this position. To get into a woman-on-top position or doggy-style position, let alone anything more adventurous, then seems really 'naughty' and she may have a fair amount of trouble getting over this mindset enough to truly relax and enjoy the ride (no pun intended). You two need to talk about how she feels about different positions, and if her discomfort with them extends beyond just not knowing what to do, then you as her lover will need to be very supportive and patient to help her explore the wonderful possibilities that sex has to offer. Be communicative and let her know (a) how much you enjoy what she's doing, and (b) what variations might make it even better for you/her.
Oh, and one more thing: one of the most important parts of any position happens before you actually get into that position --- it's called foreplay. She'll have a much easier time getting over inhibitions and inexperience if she's really aroused and feeling secure and loved, so make sure you spend plenty of time on the 'warm-up'. Also make sure that if you don't make trying 'strange' positions into a major focus, since that will make her/both of you less comfortable and the whole experience will be less natural. Rather than going into things saying that you're going to try such-and-such a position, let it happen naturally (after you've discussed the possibility of it happenning, during a moment when you're not involved in sex).
(Let me be clear here that I don't know if any of this applies to your particular situation or not --- it's just something that you need to consider. Please don't be upset if it isn't applicable. What I say here may or may not help you, but if not, it still might help someone else who happens across this thread later, so I'll say it anyway...)
Hope some or all of this is helpful to you... I'm sure the rest of the Pixies' gang will have more to add, so I'll step aside and let them! Good luck with everything!
--- sweetstuff