Adsense: How to select which ads are displayed?

Hello,

I've got a nice WSO about adsense, however it doesn't cover the basics, and I would like to understand a little better how adsense works


Lets say that I just found a nice keyword that pays a lot per click in ad sense
And I want to create a site optimized for that keyword, I will use a related article and then display the adsense banners and links.

The thing here is, how to configure adsense.

First of all I already have an account that I've used for some of my sites (sites in spanish) but now I will be doing keyword sites in english, do I need to create another account?

Second. Lets say I already have the website ready, I read that adsense automatically select the ads and links to show based on your website content.
How can I be sure that the displayed text links for example are the ones for the keyword I know., is there a way to select the keywords directly?

Third. There are some options in the adsens ad creation site, like making groups and stuff.. should I use that.. or just create the ads withoug setting up all that stuff..

Any basic help with adsense about this is really appreciated!