She has not masturbated before, which I think could be a problem
Bingo. I think it's important for her to know what 'gets-her-there' before she'll be able to coach you to make it happen. Also, unlocking the sequence (yes, chicks are like Escape Rooms) will be amazing because then, you can simply rinse-and-repeat as necessary.
Other than that, my typical advice is as follows:
The best advice I can give you is slow down.
You want the it to be a "sexual experience" not sex.
When she masturbates, what does she do? Was it penetration or clitoral stimulation? Knowing which pleasure zone is the "right one" is critical. Next, when she 'fantasizes', masturbates or reads racy material, what gets her juices flowing? You're going to have to make a study of her sexuality if this is truly your mission. Also, don't be afraid to think outside the box!! Does she like gay porn? Gang bangs? Role Play? Youth doesn't necessarily mean she doesn't have a fully kinky and mature sexual palate.
Don't be afraid to incorporate her favorite toy into your sex life. At the risk of seeming a bit cliché, they're called marital aides for a reason. On that note, you could encourage her to figure out her own orgasm privately. Part of her knowing how to cum, is just that, knowing what needs to happen to her body to climax. Some girls are like checkers - others are a fucking Rubix Cube!
Did I mention slow down. In my experience (I'm 52 - been married twice - spent 5-7 years as a "swinger" and have had multiple [even simultaneous] partners) females tend to be more of a slow-burn as opposed to an explosion. The hard to crack cases may require more patience than the typical 21[M] is prepared to have.
First step - YOU SHOULD MASTURBATE BEFORE BEGINNING THIS PROCESS. Take your sexual tension out of this process.
- Fully clothed making out - 15+ minutes! Yes 15 minutes (seriously, look at a clock before you start). It might seem like an eternity, but heavy necking is one of your best weapons. Fully clothed. Be gentle. Back of her neck. Eyelids. Shoulders. Jawline.
- Don't let her touch your crotch or dick either! She can't touch you - be in charge!
- Avoidance of erogenous zones for the first 30 minutes (½ hour). Crazy talk I know, right!? Listen, I can say easily the majority of females I've known enjoy the 'anticipation' as much as anything! Come close to her nipples but don't touch!. Come close to her labia but don't touch. Not even on the outside of her clothing.
- Focus on the 'lesser' erogenous zones; hairline behind the ear; the ear; the eyelids (gentle kisses); caress her face; knead her hands, lightly gnaw on her knuckles (gently).
- When you're feeling like "Dude, I can't fucking take this a minute longer (and neither can she).", rub her feet! Use lotion. Deliberately make it the sexiest foot massage you can imagine (better than porn). Work her calves, but stay below the knee!
- Now have her get naked; you can remove your shirt, but your pants stay on. Make out more - NO erogenous zones yet. Tease her by getting as painfully close to hers as possible. Grab her ass. If possible, press the inside of her thigh in such a way that her labia are parted! Let her feel your erection against her body, but she CAN'T TOUCH IT!
- Talk to her about how erotic this is! Fucking talk to her dude. Tell her how hard your dick is. Tell her how it's dripping. Tell her how much you're looking forward to what's next. If she likes it dirty, be dirty.
- Full body massage - which for her will be more like torture because at this point, she wants to be fucked so badly, she's probably even a little pissed off - that's good.
- Minute 31 brush her erogenous zones. Do NOT grope them. Do NOT grind them. Do NOT pinch or penetrate! Touch them as if by accident. If she responds, point out to her that you notice "Oh, you like that eh?" You're teasing her with a purpose.
- Your pants are still on by the way (and she's still prohibited from touching your junk!).
- Begin to use your mouth on her erogenous zones. Again - DON'T attack. Do LESS than she wants. I mean it. Less. If she's used to you pressing your mouth against her Mons pubis, DON'T. Instead, press your chin there, and deny the tongue. Spend at least 5 minutes [pref 10] doing LESS than what she desires.
- Now full oral with digital (of vibrator) penetration. Don't be shy now, give her what she likes best with your tongue. Teasing is done, she's cashing her chips in at this point. When she says something like, "Oh please just fuck me already." (and they almost always do) - don't fuck her! Not yet.
- Minute 45 now you can fuck her dude.
Listen to me carefully on this last point - if you follow this formula - she will cease to give a shit whether she actually climaxes or not - and is likely when it will happen.