The basics

You can move around with the WASD keys. If you press e, you'll interact with whatever's in front of you, or if you press r, you'll interact with what you're standing on. You can use the arrow keys to change the direction you're facing. The number keys will select an item in your hotbar, or you can click on an item in your hotbar to select it.

Tools

Some tools (such as hammers) are used by having them selected in your hotbar and then clicking on the world, while others (such as pickaxes and shovels) need to be equipped and then used with an interaction key (e or r as described above).