When Hammer first loads, there is no map open. Go ahead and start a new map by picking "New" under the "File" menu. Once the new file is created, there are two toolbars on the screen--one horizontally on top and one vertically down the left side. You will be using the tools on the left most frequently. The section on the right hand side is mostly for texture and entity selection. We'll cover them one by one in detail later.
Let's tidy up the various tool bars on the screen first. Drag the "VisGroups:" bar all the way to the right side (sometimes it starts out cluttering the screen, depending on the resolution of your monitor), so you'll have more screen real estate for viewing your map. The VIS groups is an organizational tool for map editing. I don't find it particularly useful, and many experienced mappers don't really bother themselves with it either. So let's ignore it for now.
After you've cleaned up your screen a bit, it should look like this:
 New map |
The central part of the screen consists of four different views of your map. The camera view is the most difficult to understand, but it's also the most powerful. Imagine you are actually inside the map, getting a preview of what your map is going to look like once it's done. You can "fly around" in the map using different camera controls, but we won't get into that right now.
The 3D camera view has several options, and you can select them by clicking on the word "camera" in the upper left corner of the grid. I find the "3D Flat" option most practical, but I occasionally use the "3D Textured" view as well. The other options are for specialized mapping tasks. Set it to "3D Flat" for now.
The top (x/y) view is the view of your map from a birds-eye perspective. It's the easiest to work with. Whenever you become disoriented, it's the best place to get you back on track. The side and front views are self-explanatory. They are important because they give you the position of objects along the vertical axis (or the z-axis). The z-axis determines how tall or how high the objects are in your map.