how do you get thicker
You mean in size? You can do it by either adding muscle mass, or eat high caloric foods with no exercise and you'll get fat.
No if you are young, you may have a high metabolism and you will have a hard time putting on weight until you get older. Then it comes easy and very hard to get off.
Well don't just eat and not exercise, you will just get fat. Try weight training and drinking/eating lots and lots of protein! Your muscles won't get bigger unless you feed them with protein.
what do you mean thicker what?