Uh, I'm not sure if I understand you, but I'll try.
Anyway, after you have made a room or something, and you want to try it out in-game, you will have to compile the map into something the engine can understand. So, if you are using Hammer, you could set it up to compile your map for you. However, I would suggest you use Batch Compiler (there is a walkthrough for BC with Steam, I suggest you check it out sometime).
Anyway, with Hammer, if you have everything set up in Tools > Options > Game Configurations and Build Programs you can use it to compile. Simply press the button in the top bar that looks like a movie strip or something (button furthest to the right), and it'll pretty much compile for you and start NS with your map.
Also remember to get XP-Cagey's compile tools, you can find them in a stickied topic in the Mapping Forum.
Oh, one more thing, for your map to run in Half-Life, you'll probably need an info_player_start entity somewhere, otherwise you might get stuck in the void somewhere.
Hope that helps, if only a little