I was seeing a video where the map creator selected a wall, then changed to clipping and, i don't know how, he selected an area from the wall.I tried that and I just cutted up the wall ._. .
That's why I need a complete guide with all the content, commands and other things about hammer.

Any help would be appreciated.

The Valve Developer Community, via the link at the top of the site under "Portal 2 Mapping Wiki". There's videos there. I'd link you to the page (it requires some, gasp, reading to find) but it's painfully obvious you haven't bothered to look at all; you'll learn more if you figure out how to find it yourself since the VDC is the place for 100% of information regarding Hammer and the Source engine.

Good luck.

The thing you're refering to is the clipping tool. When you click it once there will be a red lined area and a white one.The red thing will be cut off. click it again and they'll change positions. Click it a third time and it will be both white. Always check if both sides are white or if the correct part is cut off.

You could've find this on one of the first tutorials on the valve developer community. Make sure you read them, they help a lot! And if you have any other questions, feel free to ask them here (but usually a quick google search or a search on these forums can also answer your questions )

I always wanted to create a (video) hammer tutorial which displays all the things beginner to intermediate hammer users don't normally know.

You'll be surprised how many people don't know visGroups or the 'Group, Object, Solids' select options exist.

I think the most common problem people have with their first map is their brushwork is terrible and not very optimised. You'll find that even if the map has a lot of brushes, none of the brushes are actually func_detail, and hence, performance goes down the drain.

Tip: Don't expect much from your first map. It is a learning curve. Hammer isn't very difficult to use, it is just overwhelming to begin with.

The Valve Developer Community is excellent, obviously, and this guy has some very useful video tutorials that I used extensively last year when first trying to get my head around Hammer:
http://www.youtube.com/user/SolarChronus

The map I just put up in WIP (Double Bill) is still basically the only map I've ever worked on for any game ever, and pretty much all the knowledge I used for it was either garnered from the above two sources, this very forum or from my own tinkering about. Now I wouldn't like to comment on the quality of my own map, but it is at least functional, of reasonable length and something that I'm personally very proud of having created.

And thanks to chickenmobile I just learned about visGroups, which will make the next thing I work on much less of a pain in the arse.