Hammer is a level editor for source, you can open it in a source engine game's bin folder, if youre making your own game you can just try to use a Map Base, or HL2DM and HL2's copy of hammer.
1. Use the WASD and Arrow Keys to move the camera
2. Use the block tool (white block on the left) and draw on the grids.
3. Place down entities with the entity tool (pill looking thing on the left), place down a player entity
4. Wall up a room with brushes to prevent leaks.
5. CTRL S to save your map to make its VMF file, then F9 to compile the map
6. Load your map by loading the console (normally multiplayer games have a map picker that you can juse use)
7. If you havn't enable the console in the advanced tab in your options