I had been having sex with myself for years before I got to have it with someone else. While most of that sex involved fantasies about people and situations (e.g. me with the Flintstones, me in the Eight is Enough house, me and Raggedy Ann...I'm not saying it was pretty), some of the time I focused on trying to imagine what sex with another person would actually feel like.
Notice that what you call masturbation (a perfectly fine word) I describe as sex with myself. I do that intentionally, because I think that sex with ourselves and sex with others have more in common than we usually allow. When you have sex with yourself, if it's good, your body goes through similar changes (interest, excitement, climax, relaxation) and your mind can turn inwards and focus on how sex feels in a way that we sometimes do when we're having sex with other people. So I guess my first response to your question is to tell you that even alone, you have way more opportunities to learn about sex and be sexual than you may think you have.
But having sex by yourself isn't the same as having another person in the room. I'm sure this is a totally unsatisfying answer but the truth is that until you do take that risk (because sex is always a risk; emotional, physical, spiritual) you won't know what it's like for you.
Even after you do get to have sex with someone else, all you'll know is what that sex was like. No two bodies are identical, no two people are the same, and so no two sexual encounters will ever feel exactly the same way. If you're sober and at least a little self-aware, sex will be different with every new partner, and it can be completely different with the same person from one encounter to the next. Sex can be a cookie-cutter experience, you could do it the way you ride a bike, shower in the morning, or get ready for bed, meaning more or less the same order of things, the same thoughts, the same feelings every time. But good sex, great sex, is sex that begins without you really knowing where it will end. It has boundaries and parameters (so good sex doesn't mean anything goes, nor does it mean you shouldn't plan) but it is as free of expectations as any encounter with another human being can be.
So that was a four paragraph non-answer. But I want to try and give you an answer-answer in part because I know that being prepared can reduce anxiety, and in part because I think it's a very interesting question. After all, if it's true that every sexual encounter is different, is there anything we can say that sex, in general, feels like?
If I had to describe what sex feels like in a way that would fit for any kind of sex, I think the only description I can offer is that sex with someone else feels hot, or at least hotter. Regardless of what you're doing, and who you're doing it with, when you have sex with others there are, by definition, more bodies in the room. These bodies have heat, they have breath, they have consciousnesses; and all of the sudden the bed or car or couch just feels a lot hotter than it did when you were on your own.
The presence of these other bodies and other consciousness can have many other ramifications for what sex with other people feels like. Another person's body heat can stoke your own internal heat and draw it out of your body. It can pull you just slightly out of your own body, closer to a space between bodies where what is happening is not completely of your body or of your partner's body, but of something else.
On your own, sex can be fantastic and wild, and even unpredictable. But it's always more or less under your control. With another person (or people) in the room a thousand possibilities present themselves and each one offers the opportunity for potential pleasure and self-discovery (as well, of course, as opportunity for shame, embarrassment, violence, and coercion). I don't include those to turn you off of the idea. Just to speak honestly about sex, and remind both of us that we can always calculate, to some extent, the risks we take.
Another thing that keeps coming to my mind is the fact that we can't tickle ourselves as being relevant to a discussion of the difference between masturbation and sex with a partner.
I'm trying to imagine how I would feel about this answer if my 15-year-old self were reading it right now. I think it would be annoying, since ultimately I'm still saying that you won't know until you try it. If you're super keen to do some solo trial runs there are things you can try.
Before I ever had sex with someone else I thought if I could approximate what another person's body felt like that would be helpful. Since I didn't have access to the actual bodies that I imagined having sex with (and the way we imagine those bodies, and which bodies we imagine is a whole other conversation!) I prepared by reading smutty stories, watching porn, and using everything from lube and sex toys to the three great "f"'s (fruit, fabric, furniture). Having survived all that more or less in tact, and now having had sex with other people I can tell you that while it didn't actually help me prepare for sex with others, it was a lot of fun and made me way more sexually creative with partners. So while I wouldn't fixate on trying to prepare by mimicking sex with another person, I wouldn't rule it out either.
Finally, let me suggest that once you do have sex with another person, the first thing you should is write about it. It might come in handy sometime in the future if you find yourself talking to someone who hasn't had sex with someone else yet. I think if I had written something down about how I felt after the first time I had sex it probably would have been one word; grateful.