“Do you have this in green?”
You're out shopping for furniture with your fiance. You found a sofa that you like except for the color. You want to ask for another color, so you call over an employee at the store and ask this.
Do you have this in green?
Use "this" when pointing at an object that's close to you. How close? When you're standing up, it's usually used for things that are 4-5 steps away. When you're sitting down, it's usually for things that are within arm's reach.
You use "that" for things that are further away, or for something that's closer to the listener than it is to you:
Do you have that sofa in green?
If you and your listeners are all looking at copies of the same thing, you can say "this". For example, if you're giving a presentation with a slide show, you can talk about what's on screen:
This is a projection of our revenue for Q2.
You use this phrase when shopping to ask if the store has an item similar to the one you're looking at but with a different color. Another example:
Do you have this in white? (While holding up a shirt.)