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.