Disclaimer: not trans myself.
JSF, I applaud -- very much -- your desire to represent trans people in your writing. However, like some other posters here, I worry that the wording of your post is indicative you haven't quite done enough research yet to pull this off.
Also, rather than just having a character who happens to be trans, it sounds like you're writing an "issue book." Which is much, much harder to pull off, and also runs the risk of appearing to try to "speak for" trans people and their experiences. Because of this, you're going to be under much more scrutiny. (As you are probably aware.)
Here are the pieces of your post that worry me, as someone who has trans friends and some (small) awareness of trans issues:
the MC of the story is a straight male--eighteen
As others have said, be wary of filtering trans issues through a straight cis male lens.
finds out later that she was born a man and had the prerequisite operations in which to become a woman.
I know you explained what you meant to kuwi, but it worries me also that you worded it this way in the first place. She was born a woman, was a woman before the surgery, and was a woman after the surgery. Full stop.
The bulk of the novel deals with him coming to terms with his growing feelings of attraction for her and his acceptance of the situation.
This feels rather trite to me -- as if that's the only way a cis person could end up falling for a trans person. See below:
He also has to deal--naturally--with the opinions of his peers in school, his parents, as well as questioning his own feelings.
It worries me a bit that you think EVERYBODY EVER would "naturally" struggle and question his own feelings about fancying a girl who happens to be trans. Maybe the MC in your book does, but I don't think that's at all a given in more progressive social groups. In fact, the subject of dating someone trans came up in one of my groups of friends and everybody just sort of shrugged and said they didn't see it as an issue (warmed my heart, honestly). It came up in discussions with another group of friends and only one girl said, "I don't think I could do it -- I feel bad about it, but I just don't think I could." Everyone else at least said they'd be open to it and most said they thought it a non-issue.
Yes, my friends have selection bias, and I think probably if you polled everyone everywhere you would (unfortunately) find that many people have issues with trans folk. And yes, trans folk (obviously) face prejudice and I'm not saying not to include that in your book. But it bothers me that you seem to consider this sort of "grappling with her trans-ness" state to be the only possible course for a cis person to fall into a relationship with a trans person -- it rings alarm bells for me that you might be writing to a stereotype here.
To tangent for a moment -- you know what I would really like to see in fiction? A trans person who is incidentally trans, and dates people like any other person in the cast, and is informed by her trans-ness, but the book is not a Trans Issue Book. Obviously, that's not the book you want to write, J.S.F., but I'm just mentioning that I'd like to see it.

(Check out the way the webcomic Questionable Content has been handling Claire's character in recent strips for someone I think is (so far) a great example of a character who is a full human being and is informed by being trans but is always treated as a normal woman. In fact, the creator's lately been ship-teasing her and the main character (who knows she's trans and is perfectly unbothered by it), and there's no indication that they'd be any less likely to get together than any of the other characters.)
My question is whether I should reveal her changeover near the beginning
It still worries me that you're referencing the reveal of her being trans as the reveal of a "changeover" -- as if she was born a man and then "became" a woman. Being trans isn't about surgery (and some trans people do not opt for surgery). It's not defined by one moment in time in a hospital. How would you have worded this if your FMC was pre-op? You could still be writing the same plot, of a cis MMC falling in love with a trans FMC. She would still be trans, and still be female, but there would be no "changeover" to reference.
I guess what I'm trying to say is, your post comes off as thinking her femaleness is all about the surgery, which is not a good place to be writing about trans issues from.
or have it gradually introduced with hints along the way.
What hints? I'm assuming you mean stuff like references to how other people treat her, or how she reacts to a trans joke, or something like that . . . I don't think you mean "hints" as in, "hinting that she's somehow masculine in some way," but just making sure.
I do not want to make light of transgendered people or minimize the problems they face with being accepted in mainstream society, but at the same time, I don't want to preach. I would like some honest opinions on this and would be grateful for any and all feedback.
Here's my recommendation, for what it's worth: Make the book about something else. As in, make the main plot something different, and have the romance, and your MC's struggle (or not) be part of their lives in the context of the larger plot. Because if you make this a book about "being trans" or "loving a trans person," you're making the trans person's life entirely about being trans -- rather than that person having other goals and motivations and conflict in her life, as all people, trans or cis, do. And I think that's one of the main pitfalls people fall into in writing about marginalized demographics -- making the XYZ character be ONLY about being XYZ, instead of, y'know, concerned with all sorts of other life stuff. I feel like a romance complicated by your MMC's ingrained prejudice against his trans love interest would be far, far more interesting, and far, far less likely to be problematic, as part of a larger plot about something else.
Whatever you decide to do, I recommend a LOT of research and a LOT of reading. Read as much as you can by trans people and lurk in online places where trans people talk publicly about their experiences. Find more than one trans person who will beta for you. Again, since you're writing an issue book rather than an incidentally trans character, you're going to be under a tremendous amount of scrutiny here, and as I'm sure you agree it's one of those things that's very very important to get right because there
are so few positive portrayals of trans folk in literature.
This is just my 2 cents. Thanks for being open about hearing other opinions. And again, I'm not trans and don't feel I have any authority at all in speaking about the trans experience, so consider what I've said in that light. A lot of the stuff I've talked about here bugs me when something similar applies to a different marginalized demographic that I *am* a part of, or comes from attitudes I've picked up from reading what trans people say about themselves. But if someone from the trans community or someone with more knowledge of trans issues comes along and tells me I'm wrong, listen to that person instead.
