(Double posting because i dont know what the text limit is)
Allright, now that Hammer is open, click on tools, and options. Click on the "Game Configurations" tab. Click the "Edit" button thats below "Build Programs", select "add", name it Half-Life, hit OK, then close it. Select that config., then click on "Add", and find/select Half-Life.fgd. Skip down to Game executable, and select your exec file (example would be C:\Program Files\Steam\steamapps\ninjabunny0\half-life.) Then Mod Directory (same as the exec directory but with \valve at the end), Game directory is the same for Half-Life, and rmf directory you can leave alone, so it should look something like this
Hit apply, then select "Build Programs". Select the HL config, then fill in the blanks (the .csg, .bsp etc are part of Zoners tools.) Make "Place compiled maps in this directory before running" your half life directory with \valve\maps at the end. Hit apply, then select "textures." Select Add WAD and add half-life.wad. Hit apply, ok, then File->new. Now its time to make the actual map.
First of all, this will help. On the upper left corner of the upper left box, where it says "Camera", right click on it and select 3D Textured. Now, on your left, thats everything you need for blocks and entitys, clock on the "Block Tool" (it looks like a white cube). now, look to your right, there should be a "browse" tab. In there, you can browse through your textures. Select any texture you want to be a floor (example would be C1A3FLOOR02). On the "Top" section (upper right corner), click on the graph, and adjust it to any size, right click on it, and select Create Object, then right click on it again and carve it. This is what Hammer should look like so far (not exactly)
Now, click on your entity tool (above the block tool), and make a entity by clicking on one of the graphs, move it around to the center/a bit above the floor, and select "create Object". Right click on it and select properties. Go down the list of entitys untill you find "light". Select it, and click brightness. The value is by default 255 255 128 200. The first 3 numbers are for color, the 4th is brightness, change it to 600. Then close the window. Now place another entity at 1 corner of the floor, create it, properties, then find "info_player_start", then close the window. Make another entity at a opposite corner, make it a monster_barney. Then place 1 final entity anywhere on the floor, and make it a weapon of choice. Now you have a simple map with something to kill. Adding walls in this map is only optional, but you better get the hang of making it. Now press F9, and it will pop open a window (if you havent named the map yet it will ask you to name it.) Keep everything as it is and select OK. It should compile the map, start up Half-Life and load the map. This is a very, very small map and should take no time compiling. And there, your first (but by far from a real) map.