Welcome, 0TENDOUSOUJI0.
Most people seem to use 6-8mm foam. You'll still see some great looking suits with as low as 3mm, even though you'll find that it's pretty flimsy, and some daring builders using 12mm thick floor mats, which doesn't flex as easy or give as much room inside the armor, but is much more sturdy.
Difficulty level is different for everyone. Starting with a simple piece like JFcustom's Captain America helmet is a good project for beginners and will be an easy way to get a feel for foam building and see if you're up to it. JF templates are great for beginners in general because it makes building a lot less complicated and quicker. Some can get away with speed build times for a full armor as low as 6-8 days, but some take their time and may take a year for a full armor. It depends on how much time you have available and how long you want to put into it to make it look really nice.
Hope that helps.