Typically used are waterstones, which produce anywhere from very little to a lot of a muddy abrasive slurry. There is a lot of information in the forum on how to use them and what to get, The cheapest option is a combination stone, typically 1k/6k grit, but there are other options.
Then you need something for truing, and understand that ordinary grooved steels just ruin your knives. Some will just strop their knives on their high-grit waterstone. I have a fine Arkansas that I rounded one of the edges for this purpose, it removes a little metal but is more like a polished steel, and more convenient, though quite arguably not as good as stopping on stone of some form of fine abrasive charged strop. Something like an Idahone fine ceramic steel is probably the most popular [good] option.
An Idahone and full-sized (2.5 x 7.25") combi will be as little as $80, and is all you need to get started, though you probably should have a 4-500 grit stone for thinning, which your knives are likely in desperate need of at this time.