If the woman initiates, there is nothing to decide, you just do it; however it is more likely the man will be initiating -- but relax, it's easy, just do the following until she says "Stop" and then go no further. If she never says stop, then it happens!
Hold her hands and look at her. Compliment her on two or three substantive things that you like about her. Tell her how she makes you feel. Tell her how she fits into your future. Kiss her lips. Hold her. Kiss her on the lips again, then her neck, her shoulders, and back to her lips with your hand brushing her cheek. Casually brush your thumb from her face and neck across her nipple as you move your hand from her face to her back and hold her tight. Kiss some more. Feel her breasts over her clothes then under them if she is willing while continuing to kiss her on her lips, then expose her breasts without completely removing her top. Kiss her chest between her breasts, under them, around them, and next to her nipples, teasing. Blow on her nipples, wet your tongue and lightly lick one nipple while caressing the other breast with your hand.
Slowly move your hand from her breast down her side and across her stomach just above her waist line. Continue licking her nipples while slowly reaching down between her thighs. Lightly brush her vulva with your fingers outside of her pants/panties for a minute or two while kissing, then reach inside and penetrate her with your finger -- she should be wet enough by now -- alternate kissing on the lips and licking and sucking her nipples.
After you have fingered her a while pull off her all of her clothes completely and take off your shirt. Press your wet tongue flat up against her labia with the tip of your tongue on her perineum. Lick upwards in 3-4 looong, slowww gentle strokes. Increase the speed and pressure gradually every 4-5 strokes until your are lapping her like a mad dog. When she is ready, rip off your pants and climb on top of her.
If you are not hard for some reason (you may not be the first time as you are nervous and thinking about pleasing her), direct her hand or face toward your penis so she can jerk you off or start blowing you. At this point she may dive in and start blowing you as soon as she gets the hint about what you want her to do, but if she is inexperienced you will probably want to show her how to use her hands instead as a hand job is practically a no-brainer.
Once you are hard enough to slip it in -- even semi-hard will do if she is wet enough (she will be) -- slip it in. You don't need to do anything fancy the first time. Just plow her until you ejaculate or lose your erection and you'll both be plenty happy -- even if that happens faster than you wanted. If you don't ejaculate the first time, you can let her get you hard again and keep going or just relax and try again on another night -- it gets easier every time.