Where to start? That was a question I pondered for too long, and I finally made the leap by buying an almost ready to run model and started tinkering. That doesn't mean that's what you need to do, but it is one possible avenue. There are alot of knowledgable people on this forum that can guide you down the path. Some guidlines to help you out.
1. Depending on your location, (I will assume USA) you need a suitable radio set and it cannot be 2.4 ghz. You need a 75 mhz set with at least 4 channels, 6 is better, and 8 is great.
2. You need a model. There are some great ones out there. Precision Patterns produces a nice 594 class Fast boat for just under 300 dollars. Matt over there is a gifted model kit builder. I personnally have one of his Older 637 Fast boat kits, and a rare Thor 688 Fast boat kit which I am going to start this week. If nukes are not your taste, you can convert a revel gato or type VII uboat, although revell has discontinued these models and the price is going up. usually about 150 dollars. OTW makes gorgeous models, but the price is alot higher than other venders. Submarineworks makes an excellent Ohio class Trident boat, or even the SSGN conversion of it, and he has the only 1/96 Virginia class model (albeit it is the pre-PSA non mip boat still an excellent model) Pete over there can help you there.
3. You need to buy or build a wtc for your electronics. Need a brushed motor, an ESC, and some micro or mini size servo's.
4. You need to decide on a ballast system. You have Dynamic diving, where the boat is positively bouyant and you use speed to dive the boat, or static diving where you can submerge without speed. There are several types of these static systems. RCABs (recirculating air ballast), RCABS-R (a different form of RCABS), Propel Backup (Floodable ballast with propel gas blow), LP Blow (SNORT on some other forums, but thats the funky British way to say it (hope I don't offend)
) where you use an air pump to draw in outside air to evacuate the flooded ballast, and there are other methods out there that get more elaborate.
5. You need to ask questions if you get stuck. There is no such thing as a stupid question, wait yes there is, the question not asked is the stupid question. Tap the resources on this and other forums.
6. you need to get it wet, and don't be afraid to chase your model into the water. I have dove in after mine on more than one occasion.
7. Don't let the petty bickering get to you, don't listen to it. it is on all the forums, I ignore it and talk to the experts and learn what I can. Been in this for three years now and I love the hobby, and the friends I have made while doing it.
If you do American Nuke boats, I have served on them for over 20 years, still am. I will let you know whatever you want to if it isn't classified.
When surfaced, the bridge access hatch can be optionally open or shut, however, when submerged it is required to be shut. (Really a no s***t precaution from a procedure in use in the sub fleet today.)