Hi there
puckkeeper34 ! Welcome to the 405th! We love first time builds and there is a very strong Marine gang here.
It seems like you have chosen which suit to make which is a first step so congrats! The next step is deciding what material to use. The most common medium here is eva foam. Other popular options are fiberglass/bondo, and 3D print. I would suggest watching some videos on youtube and checking out our "tutorials" section to help you choose a method that works for you and your resources.
After that you should check the "Armory" tab at the top of the screen. That is where we keep all of our free files for various armors. Idk what's in there for marines bc I haven't made one but it's worth a look. If you can't find what your looking for, I'm sure one of the active Marine gang people has some files you can use.
After that, you just pick a piece and start working. Some people say start with something small that you can remake easily while you are learning. Seems logical, personally I like to start with something fun/wearable. That way you can wear it for motivation once it's done. It doesn't really matter what you start with, although some pieces are traditionally harder than others.
Tools you will need and cost of the build will depend on material chosen. You can certainly pay someone to make the suit for you but that's not as much fun and frankly is not likely to be cheaper. Time depends on how much time you have to commit to building every day and what level of detail/ perfectionism you are going for. Some people crank out a suit in a few months, others it takes a few years. The Marine suit doesn't look as complicated as say a spartan suit so I don't think it generally takes THAT long.
Idk about where to get the BDUs for under the armor but I think people just buy a camo set of BDUs that they like without worrying about it being specifically from Halo. Idk, I will have to refer to someone who has made a marine for that one.
Building a suit of armor from scratch IS a daunting task. It's a lot of work and usually involves learning some new skills and buying some new tools. It's a big undertaking which is what makes it such a rewarding project. A lot of people stop, run out of time, or lose motivation and have to decide if they want to continue. It's not something that's simple or easy to do. But if it's something you want to do it can be a very fun and rewarding process.
This forum is probably the best place for support and encouragement on the entire internet, so you're in the right place for taking on such a challenge. Ask questions, read other threads, watch videos, and don't be afraid of mistakes. Some people rebuild parts over and over again.
Good luck and have fun! We are here to help!