Hmmm... not the worst start i've ever seen but:
1. if you wanna have cubemaps, you have to place them in the editor, too. (env_cubemap)
2. Your light_spot is working, but you just need to align it with the lamp and a env_sprite wouldn't be the worst idea too.
3. ATM there is no reason for the skybox beeing that big, and there will never be a reason why it is not useing the ground.
4. The pillar and the strange triangle part on the wall (which both wouldn't make any sense in there anyway) should be at least func_detail to reduce the vis leafes.
This is an very important thing you should remember: keep the struckture of worldbrushes simple (always think in sqares) and do the detail work by useing func_detail (build them with brushes and tie them to func_detail after you finished building them).
5. this warehouse is more a HL2 setting then a GES one, don't use to much HL2 props.
6. before you even start with making a real map, think about it... maybe draw some ideas down to paper and always keep in mind, it has to be suitable for at least 16 players.
Next start to do the basic brushwork, and don't care about textures, lights, props, effects and details for now.
If the basic brushwork is done, place some player, ammo and weapon spawns and have a first testrun with a few ppl.
Listen to their feedback and suggestions and think about it (again).
If you're satisfied with the flow, you can go for the details and other stuff.
BUT FIRST OF ALL, LEARN THE BASICS OF MAPPING BY DOING TUTORIALS!!!
resources:
http://www.interlopers.net/tutorials/http://developer.valvesoftware.com/wiki/SDK_Docs