I'm going to answer your questions one at a time. I hope you don't mind.
I have not ready anyone else's answers, that way I can give you my unbiased opinion/knowledge.
1)i really want to go swimming at some point but my boyfriend will only go if he has a one piece suit. However he doesnt want to go put shopping for one and refuses to let me buy one for him. What do i do? There are websites where he can get a swimming binder.
https://www.gc2b.co/pages/gc2b-binders is one. This way, he can wear the binder under a t-shirt and a pair of swim trunks, many men do. I don't think he would want to wear a women's swim suit, that's the only other one piece swimwear I can think of.
2) a really wired thought came to me for our parts which was is it possible for me to give him my male part and me his female parts? Doubtful. You'd have to have an exact DNA match. Its difficult to find that even in siblings, due to the way DNA mixes even in families.
3) what parts of clothing should i start wareing out in public? Cause i do wanna ware female clothing but not overly female clothing and id like a starting point just so i can try it or get used to it.Wear whatever makes you feel comfortable.

If you want to wear jeans and a blouse, do so. As long as you feel comfortable and mose importantly
safe, there's nothing stopping you but you.
4) would i beable to bare children and by extension our children if we froze our gametes and such?You won't be able to bear/carry children, but he would be able to.
https://en.wikipedia.org/wiki/Thomas_Beatie is a man, yet had three kids.
5) are the effects of hrt greater or quicker at a younger age?The younger you are, the more likely your growth will be for the gender you should have been born. If your boyfriend hasn't finished growing and goes on testosterone, chances are he'll end up taller than if he's pre-T. If he's done growing, then his height is probably whatever it is now. The same for you, if you're on HRT and still growing, your height will be more than likely be whatever is genetic for women in your family. Everything else, such as facial hair, muscles, and whatnot will more than likely change according to the HRT the two of you go on, despite your age. Even if you transition late, (like I did, I started transitioning at 54), you will more than likely have your body change. If your voice has already dropped pre-HRT, then it will not change without vocal training. If his is still high, pre-HRT, then it will drop once he starts testosterone. You can see where I'm going with this, right?

tl:dr Once you're both on HRT, your body will change, according to the hormone/s you're on.
6) i find it awkward to tell people im trans so would having a few stickers and badges of the transgender flag help?As long as you're safe, there's no reason to hide it. OR, you don't have to tell anyone outside of your family, your doctor and your school, and even the school is iffy. Once you change your name and gender legally, if you choose to not tell people (unless you're planning on being intimate with them), you don't have to. Its called "going stealth." It's not a bad thing, it simply means that, "unless needed, its not necessary to tell anyone." I'm sure that cis people don't go around shouting," I"M CIS!" to everyone, why would you need to tell everyone you meet "I"M TRANS!"?
Or, you could simply get things in the colors of the transgender flag. I have ADHD, so I have a fidget cube that hangs from my phone case in the transflag colors. Or, a pin that has the colors on them without the actual flag.
Again, you do what's comfortable and most importantly
safest for you.
Hope this helps! Good luck you two!
Ryuichi