1st, realize that making games is not the rockstar job that lots of people make it out to be. The game industry keeps itself alive on the blood and sweat of people who love making games. You will work 10-14 hour days for weeks at a time (or years, as some cases may be). You probably won't get a bonus when the game is done. You probably won't be paid very well. You'll have to work with people who think they are god's gift to the world, and they will probably be your boss.
But, you will also work with really cool, creative people. You will get to do something you (hopefully) love to do. You will get to create games. If this small paragraph sounds better than the larger, first paragraph, then by all means, pursue a career in the game industry.
So... the nutshell I can come up with at 4:45 in the morning (yes, I've been working since yesterday morning).
Get a day job first. It's a different path for everyone, but odds are you won't break into the industry anytime soon (it took me a couple years). You might be able to get in as a tester or intern, but it's almost as hard to make the jump to the dev side as it is to just break in.
Grab the whatever latest version of Unreal2KX XMP Super Mega Championship Edition is out. Epic has done a good job of marketing their engine and tools to devs, and a lot of places have picked up the Unreal engine and it's editor, UnrealEd. This can give you a slight advantage just because being familiar with the tools can be a big selling point to some companies.
Next, learn how to use it. Not just part of it, all of it. How to make and import textures. How to make and import meshes. How to make and import sounds. Even learn the basics of unreal script. You may not be actually creating art assets/code in the position, but as level designer you are where 'the tires hit the road'. Everything has to come through you at some point to go into the level so you have to understand everything that is going on behind the scenes.
Make a few multiplayer maps of whatever flavor you want. Focus on a few key areas:
1. Look and feel. No BSP holes. No meshes intersecting each other at weird angles. Everything lines up. There is a good sense of 'space'. Lighting is good and reflects the mood appropriately but isn't overboard. Textures are aligned properly.
2. Wiring. Doors open and close when they are supposed to. Switches work the 'right' way. Events happen when they are supposed to. Areas are zoned or antiportaled correctly.
3. Gameplay. For learning, I put this last at this point. These maps your learning how to use the editor and trying to make them look as good as possible. In general, in gameplay the player shouldn't get lost or stuck anywhere. The next area to explore should be obvious. Paths are clearly marked. Framerate is good at all places in the level.
Now, you need to make some single player experiences. You probably won't get a job making multiplayer maps (I've never made one professionally) so you need to be able to create good single player experiences. This is the hard part (learning the tech just enables you, this is the actual work!). Even moreso than understanding the technology, you have to understand the game you are making and understand the game design.
To learn how to create good single player experiences, don't just play other games, analyze them. Watch how they create tempo and how the flow of the level works. When is the player challenged? How often? When is the player rewarded? How often? What types of challenges are present? How difficult is the game? Why were certain game and level design decisions made for that game? How would the level design be different if the character could jump twice as far? Shoot twice as fast? Once you start playing games with these sorts of questions in mind, you'll start to have a better understanding of what it takes to create a level. It will take awhile, as long or longer than it takes to learn all the tools. I'm still learning and I've been making levels since Q2 days.
Good luck.
