that depends a lot on the protein, but the basic building blocks are long strings of amino acid (I always imagine them a bit like shoe laces…) that will have to fold into a certain structure. and that structure can be very variable. Like with a shoe lace, you can fold it up into layers that lie on top of each other, you can twirl the string around itself, or you can even make knots into proteins!
You can then combine several folded-up strings to make quite complicated structures. My personal favourite is a protein complex called GroEL/ES
There is a cartoon picture of its structure here:
It basically forms a basket structure, with a whole running through the middle (the green, yellow, and red bits) but it also has a little lid (the blue string) on top. It’s chaperone that helps other proteins to fold. The lid goes off, proteins go into the basket, the can fold there without being bothered by other proteins around them, then the lift goes off, and – like a magic hat trick – a properly folded protein pops out.