Indexers catalogue the files available on usenet and typically provide NZB files that help you request the files you want from your provider of choice. It's possible to get by without using one, but at first you're absolutely going to need one.
You can start off with a basic usenet search engine such as Binsearch, but you'll want to use a indexer early on
Like providers, there's a fair amount to distinguish indexers but at first, it's best just to pick one with a free trial to get started. Again, there's a list below