Most likely performance anxiety.
You need to relax.
1. tell the girl it is your first time and that you are nervous. that will take some pressure off
2. appreciate the moment, give her a massage for a while, explore her body, play with her boobs, rub her vagina slowly, smell your fingers if she isn't too diseased. ask for a BJ like another anon said.
let her give you a massage for a while.
the more contact and time spent naked with a girl, the more comfortable and relaxed you will become.
3. practise, as much as possible.
not just sex, but as I described above, being naked with a girl, playing with her, her playing with you.
the more you do it, the more relaxed you will become.
4. concentrate on the sensations, DO NOT think about your dick. think about her voice, her moans, the feel and the smoothness and the warmth of her skin, the smell of her pussy, the softness and the heat of inside her pussy, how her lips feel brushing your skin, how her breast curves and her nipple sticks out.
Aside from that, don't fap for 2 weeks beforehand, don't edge, don't ejaculate, nothing.
You will be quite horny by then, gives an advantage.
If you cum fast, that's normal, just say that was great, give me a few minutes and we can go again.
don't worry, don't make a big deal, just relax and enjoy and go with the flow.
it is important to follow your instinct, on what you want to do. don't spend much time thinking about what she wants or if it feels good for her. more importantly you do what you want, what comes to mind naturally.
Avoid porn completely, read the website "yourbrainonporn". Porn can desensitise you, and also condition you to be a voyeur, rather than an active participant. This is the first thing you should do, and stick to it.
Read about testosterone boosting foods and lifestyle habits. Can make a big difference.
Sunlight, strength and lower body exercise, dark green vegetables, nuts, seeds, lean meat, eggs, mushrooms. Avoid sugar, avoid fried foods.
Keep trying, you will succeed if you keep trying.
If you can fap and have quick, hard boners when you are alone, then you just need some practise and what I've written above.