thats a very broad question, theres a lot you should learn. read some threads and figure out what materials you want to make it from. once you pick a material, watch some youtube videos on how to use it.
Get lots of replacement blades as foam dulls a knife like nothing else. Invest in some super glue (I recommend loctite liquid but to each their own) as well as hot glue for reinforcing seams. There’s a few videos on YouTube that show how to make beveled edges and how to fill seams as well. Download pepakura designer and find the armor you want to do in our armory section and download it into pepakura.
Welcome Chungass! The first thing would be to get some unfolds that will allow you to make foam patterns. Unfortunately it doesn't seem there is any in the armory (yet), but if you do have Pepakura or Armorsmith, you may be able to unfold them yourself.
What sort of budget are you looking to start out with? At its very basics for making foam armour, all you need is a blade, hot glue and paint. If you want to get more advanced, you can use a knife sharpener/replacement blades for clean foam cuts, a rotary tool to clean up any mistakes and add detail, contact cement to adhere foam together, and a sealant (such as PlastiDip, Leak Seal, or Mod Podge) to fill up the pores in foam before priming and painting. A heat gun is also super handy to shrink those pores beforehand.
When I was getting started making foam armour and props, I primarily watched SKS Props, Punished Props, Evil Ted Smith and Kamui Cosplay. They are great leaders in the cosplay community who can give you a good start.
If you have any more specific questions, we're happy to answer them!
Edit: Make sure that, if you are using chemicals like contact cement, or burning or sanding foam, that you use a respirator and have plenty of ventilation.
Watch youtube videos from Punished Props, Evil Ted Smith, and Kamui Cosplay, (there are many more too). They really helped me start out, especially Punished Props as they give extremely detailed tutorials on even the smallest things.
Also, be ready for lots and lots of time. Every step takes a long time, from cutting out paper templates, drawing them onto the foam, cutting each piece, gluing, dremeling, heat gun sealing it, then plasti-dipping and spray painting. It all takes a long time. I'd originally started with smaller foam armor/props before tackling my armor set and it takes forever. Definitely not something that is going to be one and done especially if you have other things on your schedules such as work or school. But if you keep working on it and don't give up it's definitely worth it in the end.