My goal is to pass the time.

Although I wouldn't mind doing it as a source of income, but I'm too lazy to bother with deadlines. I'd much rather just open paint whenever I feel inspired and throw something down than to be forced into it.

Whenever you start out to pixel something generic like rocks, trees, grass, water, or anything you come in contact with on a daily basis, the key is observation observation observation.  If you're not careful, your brain will play tricks on you when you start out to make tiles and say to you "water is blue", and you can end up making a plain blue square and call it a water tile.  Instead, think about the properties you see in water everyday: transparency, translucency, reflection, refraction, the way light interacts with it, the clarity of water, things you might see in or around a pond or river, etc.  Think about the kind of light your tiles are in: is it brigt sunlight?  which way is the light coming from?