Start off by creating a playable area, for example a room. Make sure it's sealed off from the outside and you have at least one entity inside (for example the player start or some lights). This becomes hull of your map.
The four boxes are your viewports, they should be set to 3d, top, side and front so you can see what you are doing. The bar on the left containing lots of icons is what you will be using.
For example, the brush tool will let you create new objects such as crates or walls. The clip tool lets you cut them up, allowing for ramps or curvy structures Make sure your structures are planar and convex at all times or your map will not work.
The interface will take a lot of learning so youtube and google is your friend. I recommend some other software too:
J.a.c.k editor (way better than hammer, less buggy, more features, has a good user manual etc): http://jack.hlfx.ru/en/download.html
VHLT (use this to compile instead of quake tools, also provides useful tool textures): https://forums.svencoop.com/showthread.php/42174-Vluzacn-s-ZHLT-v34-Update