I'd start by taking your favourite camera out and about and start snapping photos of real textures you like the look of, things like:
- Grass, bushes, twigs, trees.
- Fences (or other wood paneling)
- Mud, dirt, patchy grass etc
- Stone walls, pavement, asphalt, sidewalks, bricks.
- Water, puddles, lakes, rivers, etc.
Take 5 or 6 pictures of each, because what it looks like on your camera screen isn't necessarily how it will turn out in the game.
Now you can load it into photoshop and give it some tlc. Alter any things that look out of place, like if you want a total grass texture you should remove any patches, light areas, dark areas. It should be mostly uniform as it will be tiling over an area and anything that stands out will break the illusion of it being "all natural." During this step you can also make the graphics look less "real" and more arty if you wish.
Next you'll need to take your favourite pictures of each texture and create a seamless tileable texture. There are many ways to do this but by far my favourite is to use the Texture Maker tool. It may seem a bit daunting at first but it's really worth learning, it can do some incredible stuff! Including animated textures (eg, for water). But don't do this for things like cars and people, they will not need to be tileable.
Also, you need to take into consideration the orientation of your camera in the game, will it be as your example a Top down camera? A side scrolling platfomer or a 3D environment? As the basic needs of each graphic will change greatly depending on your type of game.
After you have done all of this, you'll need to start mapping it to your elements within the game itself, as I don't know which kind of game you're going for I can't give anymore advice. But if you need some I'll be happy to help.
- Take picture of what you want.
- Edit picture to make uniform.
- Make tileable. (This must come after any edits, if it has anything that stands out when tiled, go back a step and edit it in Photoshop :)
- Map to environment.
Remember to always save copies of everything in case you need to go back a few steps if something doesn't go as you planned, especially if you're using 3D models, I remember having a hell of a time having to rebuild something that I didn't save :D
Hope some of that helps :)