Welcome!
Fusions: For just starting out, you don't need to think too hard about how this works. It's basically, take two personas, mush 'em together, get a new one. There are two ways to fuse personas at the start: you can either pick the two personas to fuse (choose one, and a list of potential results will show up indicating what second persona will lead to what result), or use the Fuse by Result option to pick the persona you want to create, letting the game choose the two ingredients automatically. The system is much deeper than this if you care to dig into it (i.e. you can predict to an extent which persona you'll get based on the arcana and levels of your ingredient personas, there's a chart that explains this) but for now, you don't need to worry about it.
Regarding your bonds with other characters, this affects fusion in two ways. First, there are ultimate personas for each arcana they are unlocked by taking a confidant to max rank. You don't need to worry about this yet, but it's something to think about as you decide which people to hang out with. Second, as you rank up confidants, you'll get an "arcana burst" for every persona you fuse of that arcana. This is extra experience you earn on that persona immediately after fusing it, which means you can unlock later skills for that persona early. Generally you want to take advantage of this as much as you can, but if you see a particularly strong persona you want to make and you get a low or zero arcana boost for fusing it, it's not really a big deal since you'll probably be using that persona a lot and can earn experience the normal way for it.
Conversations: For random NPCs in the world, I think generally they change about once per palace. One exception is if a character has a Mementos request associated with them, in which case their dialogue can also change after you fulfill that request. This doesn't include your party members and other confidants, of course, who talk to you a lot more often and can actually hang out with you.
Platinum: You'll need two playthroughs, one normal and one in new game plus. Three trophies cannot be earned in your first playthrough, though in theory you could earn those trophies very early in NG+. Earning all the other trophies in your first playthrough requires a fair amount of smart time management, and if it's your first time playing Persona chances are you won't be that smart. Consensus is generally to just enjoy yourself and not worry about guides if you're dead set on a NG+ playthrough; play the game as it was intended once, then minmax like hell for your second playthrough. I wouldn't call the Platinum hard, exactly, but it can be very time consuming just because the game is so long.
Endings: There are multiple endings. There are a few places where they branch. The obvious place is if you fail to infiltrate a palace by the deadline. There are also certain parts of the story that can knock you into a bad ending, but these parts are pretty obviously marked and the "right" decision pretty easy to figure out. Also, Persona is very much a game where there is a single right ending and many wrong ones, as opposed to some other games where many different endings are equally good.
Good luck, and enjoy the game!