Move with WASD keys.

Perform actions with arrow keys (once you have items loaded).

The right side of the screen lets you manage which items you're using. The top-right shows your "loadouts" - sets of up to four items you can use simultaneously. The middle-right shows your inventory. You can click an item here and then a loadout to add it to that loadout. The bottom-right is the "remove" button. Click it and then a loadout to remove the newest item from it.

Change your current loadout with 1/2/3/4 (you may have to activate "Num Lock" if using a numpad).