Vtyjkyuk

I would like to create a enumerate as following:

C1
 C1.1
 C1.2
C2
C3

Basically, all numbers have a given letter.

Thanks.

Using enumitem you can define the first level list to have label=Carabic*, which will prepend the C to the counter. for the following lists you can use label*=.arabic*, which will append .<counter> to the parent list counter.

documentclassarticle

usepackageenumitem

begindocument

beginenumerate[label=Carabic*]
 item item
 beginenumerate[label*=.arabic*]
 item sub item
 item another sub item
 endenumerate
 item another item
 item one more item
endenumerate

enddocument

If you want this to be the global setting of your document, instead of using beginenumerate[label... every time (which can be quite tedious and error-prone), you can use setlist:

documentclassarticle

usepackageenumitem

setlist[1]label=Carabic*
setlistlabel*=.arabic*

begindocument

beginenumerate
 item item
 beginenumerate
 item sub item
 item another sub item
 endenumerate
 item another item
 item one more item
endenumerate

enddocument

The first setlist will make C<counter> for first level lists, and the second will append .<counter> to the other levels.