The biggest problem you'll encounter first is the top shape of your head. There are two ways you can do this which is the cheap painful way or the easy costly way. Cheap painful way is to use cardboard to shape around your head getting the base of the helmet. Easy costly way is to buy a helmet that already fits your head.
Which ever way you choose, you then can build out from there building the ant man helmet.
Now here's where you need to choose what you want to build it with as there are many methods. For beginners or people just starting, I'd suggest either paper mache or craft foam to overlay it. For more experienced, I'd suggest using resin. Depending on your choice will then lead to further explanation.
I hope this helped.
When in doubt, just take the next small step.
With that next step take a deep breath. It calms the mind.
When the mind is calm you realize the best is still yet to come.