I've started today to use MT, as far as I've seen it is similar (but more complicated) to blogger. My first intention is to change the templates, I wold like to have one in wichh there are different categories and every post must contain to wich categorie ir belongs, what tags should i use to do this.
thank you in advance

The easiest way to do this is to check 'Category' for an archive type (Weblog Config > Archiving). If you want this to be your primary archive method, so that permalinks and such reflect the change, just:

• switch the Preferred Archive Type to 'Category' (Weblog Config > Preferences)
  
• rebuild your site

Unless you've modified the archive and perma- links in your templates, MT should adjust these automaticallly to reflect the change.

For this to work effectively, you should probably set up some categories, to have them available when you post.