Actually, hymens are a real thing, people just exaggerate about them. The reason most girls don't notice them is because that membrane will widen and thin out as the girl develops physically. By their mid-teens most girls are physically ready for sex and wouldn't feel much beyond the slightest prick or itch during their first time having sex. If you want to include a hymen breaking, the girl depicted is gonna need to be a young one, in her early teens or preteens.
As for your question, in order to be comfortable with anything, a person needs exposure to the thing, whatever it is. This does not mean that the person has to engage with this thing directly; In fact indirect exposure to things often creates a romanticized look on them.
Your girl could be thinking about how her friends sometimes talk about their sexual experiences and she could form an opinion of what she thinks she would like, or how she would feel, or what she would do, as a result. Then she could try exploring her own body in a comfortable space at her own leisure.
If you don't want to take time to show those experiences, then you can have her think back to these as experiences that have already happened to her.
The most important thing however, is that she needs to be comfortable with the person she is going to be sexual with. If she doesn't trust that person, she won't be comfortable doing anything with whoever she's with. You're going to need to show that trust in some way, if you want it to be believable.
(Yes, I know this essentially reiterated Bronx's example, but I felt maybe you'd like to have another perspective, even if it isn't much different.)