Before you start mapping in the Hammer editor/Source, I would highly recommend grabbing a cup of coffee and doing some reading first. What to read? Well, this:

Source SDK Documentation

and this:

Hammer Documentation

Now, there are many sections here which may answer questions you'd have, about performance of props, brushes, water, etc.

Many of the documented pages provide rather nice little 'tutorials' or examples that you can use when creating your maps, and it's also a good idea to look it over and get help for the various new features that are included in the Hammer editor, as well as a FAQ and tables for various types of memory usage and such.

Just spend some time and browse around: this is the official documentation, with lists of all the hotkeys for the editor as well. It's a nice place to go and check for hints and tips on various Hammer topics, and may answer many questions you'd have about making Half-Life 2 maps and Counster-Strike: Source maps.

*salutes*

-N-