Cosplay is a little different to everyone, but at it's simplest it's this: making costumes based on characters from books, movies, TV, manga, anime, and even webcomics. Some cosplayers like to act in character and make videos in character, while others do photoshoots or enter contests and compete. Some want to go professional some day, while others prefer to hang out with friends. The cosplay community is diverse as the characters they portray-there's no right way to do it or right body for it.
This page will get you on the right track on learning some of the basics for sewing, foamsmithing, and other related skills that can come in handy when cosplaying, and the second page covers topics like styling wigs, makeup and body paint, as well as photography and posing. If you're looking for help within a specific fandom, try the third page! Some fandoms, like Marvel and GoT, have active groups and resources available to help source costume pieces and crafting.
There are so many techniques that can go into cosplay (like 3D printing, goldwork, and making your own patterns), it's impossible to cover them all here. I hope this guide can be a place to help give you some starting ideas and links that you may not have considered before. Best of luck, and remember to have fun!
While you can successfully cosplay without ever touching a sewing needle or machine, sewing is also one of the best ways to get started because you can find great things at a thrift store, then modify them with a little time and patience. There are so, so many books on sewing in the catalog; these are just a few on more specific techniques that might come in handy.
There are a surprising number of skills that can be involved in creating a costume, from leatherwork to chainmail to creating custom printed fabric. These are just a few options available, but there are tons more in the catalog!
There are a growing number of books out there specifically about cosplay, which is awesome! These cover not just sewing or foamcraft but also makeup and posing techniques.