Hello hello Spartancody! i am looking forward to seeing how you carry through with this cosplay. (assuming you post updates) But as for where to start, i've no idea for a spartan, as i've never done one myself, but I can at least point you in the right direction for the ODST aspect of things. For the basics, i'd recommend checking out AndrewDFT on youtube for some guides on how to make a few of the ODST armour pieces. I've also heard good things about Kamuicosplay and Odin Makes, though i've never checked them out myself.
As for supplies, i'd recommend a razor blade or x-acto knife of some sort, and make sure to buy lots of blades if you're using x-acto, as the dull very quickly and a sharp blade makes a huge difference in EVA foam. I buy my blades from amazon. (200 blades for around $10) So, they're fairly cheap and a perfectly viable option. I've also seen box cutter/utility knifes used as well.(the kind with the segmented blades you can snap off)
For the foam, i've seen that Hobby Lobby and many craft stores similar to it will carry EVA foam in 2mm, 5mm, and 10mm varieties, as well as air dry foam clay for filling those ever so pesky seams. For thicker varieties, (12mm) i've seen them sold at Home Depot, in the format of the foam floor tiles, though it is very expensive there. (around $21) I also know that harbor freight carries them for much cheaper (around $8-10).
So, thats just an option if you feel inclined to try out of EVA foam. I also know that the armory on this site is phenomenal for reference, pepakura files, and unfolds for both foam and pep stuff to use as templates. For 3D printing, as S225 said, the Ender-3 series is a good start. I got an Ender-3 pro for around $210 and after a year, it has worked well with very few hiccups. Overall, I like it a ton and would recommend checking it out if you are inclined to buy a printer.
I hope this helped, and i'm sure that the fine people of this community would be more than happy to help with anything you need in any way that they can.