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From HalfLife 2 Knowledge Base

Learning the Basics

The basics of mapping in Hammer are real easy (depending on your definition of basics). Don't try and make your first map a spin-off of cs_office, 'cause it ain't gonna happen. Maps such as cs_office have many special traits to them. Such as the garage door, breakable glass, and all of the other features that go along with it. Maps like office seem simple, but are so complex and require much planning ahead of time it sometimes makes me sick... This is why it's important to learn the basics. Then you may move on and learn the more advanced aspects of mapping.

The First Map

Challenge
Make a map with 2 rooms. In between these rooms should be a hallway connecting the two. Also texture it and create semi-realistic lighting.
Reason
Doing this should get you familiar with the basic tools like the brush and carve tool as well as beginning to texture and lighting.

The Second Map

Challenge
Make a fun physics map. This should include features such as: 3D skybox, water, prop_physics models, ropes with objects attached, fans or rotating brushes, doors, elevators, and breakable surfaces.
Reason
A map like this will take a few days to complete if you are just beginning. Though, sometimes it may be frusterating, after learning all of these, you will most definately be ready to make semi-complex maps.

How to Learn Mapping

I find the best way to learn anything in mapping is to do it for yourself. For example, if you want to learn how to make a fan, look through the different entities. What would make sense? Hmmm, maybe func_rotating?? If not, try another one. By doing this, you are learning what each entity is responsible for. You're also developing troubleshooting skills. And if all else fails, check the WiKi or HL2World's Forums. I don't find myself asking many questions on the forums anymore though. I usually like performing trial and error. Why? I learn what won't work and what will. I also learn what I can possibly do even if I don't get the right results. Once you become familiar with all of the entities, you'll be well on your way to making complex dynamic maps.

But entities aren't the only thing used in maps, you've got to become familiar with brush work as well. This is where the fun physics map will help out. Make archways, sphere phys_boxes, spikes, doorways with the carve tool, and all that stuff. Brushwork is the basis of all maps. When this is mastered, anything is possible.

Texturing. Some like it, some hate it. Texturing has always been easy for me. Hammer makes it so easy to texture, align, and make beautiful maps without a hitch. When making your phys map, use many different textures. Learn the different tool textures such as nodraw, clip, trigger, ladder, and all of the others that may not even do anything.....Anyways, becoming familiar with texturing is easy, but is hard to completely master. A great layed out map is nothing unless it can be textured correctly.

Lighting, this one seems to give many people trouble. Just like texturing; if you can't light your map correctly, don't waste your time going any further. Learn the differences in the light entities. Learn how to use them effectively. When I first started, I thought that all you'd need for lights in a map were the entity "light". Now I found out that a combination of light_spot and light make for nice realistic lighting. Stuff like that. Here's also a good time to learn how to effectively use cubemaps.