The level editor used to be called Worldcraft. (No, not Warcraft). I think they renamed it simply to "Counterstrike Source SDK".
To make a level, well. That's a very generic request. You'll need to master the following:
2. Paths (if you want bots to be able to use it)
3. Materials and physics effects (if you want objects in the level to be affected by player actions)
4. Lighting and textures (which can take forever)
5. Map rules and setup including object starting locations and spawn locations
6. Any scripting and programming that your level will require
Among other things.
If you've never done anything 3D before then you are in for a world of trouble. It will probably take you a month of hard work to get the basics down and make a playable level. As for how to do it, there are tutorials online. I haven't done it for years and I definitely wouldn't type up a tutorial here anyway. It would take days. :-/
I haven't played CS, so here's some general advice.
Don't let the players stray from action for long. Alleyways are nice, but shouldn't be isolated for long. Have big open areas with some obstacles to fight in. And give multiple entrances. Don't make it too easy to bottleneck the enemy.
Lay out your level before you do anything. If possible, run it as a board game first, making some simple rules to test the level for balance. Even without doing this, play test often. Bring others in to play it and give their opinion of it.
Keep the setting in mind, what period is it? What time of day is it? Are you indoors or out? Lighting seems minor, but it really brings out the look of a level and sets the mood. Keep strong shadows minimal so they don't overwhelm play, but make sure they happen.
Every moment of game play should make for a good screen cap.
I don't know how cs is with weapons, so if this is irrelevant, obviously ignore it: be conservative with big guns. They should be few, far between, and put the player at risk to reach.
if by level u mean maps.. then in steam u can go to the tools tab and download the sourcesdk.. with this program u can make all sorts of maps.. pretty cool.. just google it for instructions. its pretty easy