You're doing okay for your first map.
First, instead of making a second post in a row, use the Edit button beside your name (if it's the last entry in a topic) to revise your first post.
Second, I think you'll want to put up some walls on all sides of the map. If not, when you approach the edge of the map, it will look like you're looking off the edge of a cliff.
Third, if you don't like any of the half-life sky textures, Google for "half-life sky texture" or similar. There're lots of textures available on the web. You may find a wad file to download that contains a texture you like. Else you'll have to use Wally or some similar program to create your own wad.
If you haven't already, be sure to read the tutorial on Environment
http://twhl.co.za/tutorial.php?id=6