At the beggining, as your ship just crashed, you start in the middle of the blue flat area. You are alone and there is nothing around you.This area is only a small part of the universe.
You can move in the world using the "WASD" keys, the spacebar to jump and the mouse to look and turn. Depending on the country you live, your keyboard could be different of the "QWERTY" layout. In this case, you'll could have to reassign keys in the game settings.
Blackvoxel is designed to use the 3 buttons and the scrollwheel of a standard PC mouse. If your mouse doesn't have 3 button(or you use an alternate input device), the "C", "V", "B" keys can be used instead or the mouse buttons. The "X" and "N" keys simulate the scrollwheel.
You start the game equiped with a basic construction tool called the "Constructor/Destructor". With this tool, you can grab a bloc from the world to place it in your inventory OR you can take a bloc in you inventory to place it in the world.
For the complete list of all controls, take a look at the control section of the manual.
As said before, the central blue area where you start is only a small part of the Blackvoxel universe. It's easy to get out of it by walking in straight line in any direction for only one minute.
But for starting, you'll better stay around the RTFM voxel near the starting point. That's an area where you are safe and despite the apparences you'll find here most of the resources needed to start.
Now, this mission is over and you have learned the basic controls. You can go to the next mission.