There are tutorials out there for setting up Photoshop for pixel art. There are a few steps you have to do to make sure your tools aren't anti-aliased and so on.
Other than that, just study reality and others' work, and experiment! The best tutorials don't teach you how to make specific tiles, instead they help you understand what it is you're trying to do. There are as many ways to make each kind of tile as there are artists, and if you only learn from tutorials that tell you what to do, you'll only end up copying other artists instead of making something that works for
your project.
This tutorial is a nice starting point, but make sure you read the whole thing, not just the section on tiles. You should also make sure you don't limit your learning to just pixel art - look at real rocks and grass, so that you can develop a better feeling for the things you'll eventually be depicting, so you know when you're doing it well, and what to do when you're not.
