hi, congratulation for taking that first step :) and don't worry, you won't fail. Actually, when the time comes, let it be natural; don't think about it too much, try to follow your senses and be aware of the signals that your partner is trying to send. If you are having sex the first time, trying to choose the best position should be the last thing in your mind; it's not as important as discovering what kind of person you are and how that interacts with your partner's.
My suggestion is to start slow and pick it up as the intimacy heats up, and don't forget to pay attention to what your partner's body language is trying to tell you, wither she's satisfying or uncomfortable. It's actually okay to stop and ask sometimes :)
just remember, no one should expect perfection the first time around; it takes practice to discover the gradual intimacy that you both are looking for...
and finally, you are more than welcome to try out our suggestions (http://www.sensualinteractive.com/si/input.php) once you are ready for more,
hope this helps,