Like a lot of crafts, it's gonna be a learn-as-you-go process: make plenty of mistakes, and ask or look up questions when you want to know something specific. I guess the big things are contact cement is great for joining foam together, but you'll want a respirator and a well ventilated environment to do so. A heat gun will help seal foam and can be used to soften it to make curves. A rotary tool of some kind is very helpful to clean up edges and add weathering effects down the line.
I'd recommend taking a look at Youtubers like SKS Props, Evil Ted, Punished Props and Kamui Cosplay to get the basics down.