Guide: Interacting with the environment is as simple as clicking the left mouse button while standing in front of an object. If you want to gather some wood from a tree, for example, simply walk up to a tree, position the white cursor over a block, and press the left mouse button.
You'll swing at the tree with your arm and cause damage (to the tree and not your arm, surprisingly enough). Now since rapidly clicking on things would be a fantastic way to develop carpal tunnel syndrome, holding down the left mouse button will automate the bashing action.
So holding down the left mouse button while standing in front of a tree will cause your arm to automatically batter the trunk until a nice cube of wood falls from it. You'll notice that you can keep removing chunks of wood from a tree without the top toppling down on top of you -- even despite chunks of trunk disappearing blocks at a time. It's one of the weird quirks you'll experience while playing Minecraft.