Probably simple answer

Hi all.

Can anyone tell me how to get a sequential row count field in the output of a query?

Say I want to query for all users in a table sorted by lastname and firstname.
I would like to include a column in my query called "rownum" which would uniquely identify the row in the order of the
queryresults. 

Like this:

rownum =1   lastname=jones, firstname=john
rownum=2 lastname=smith, firstname=john

etc.
I assume rownum should be some kind of function of expresion but I don't know what.

Something like:

Select ???? as rownum, lastname,firstname from users where xxx =xxx order by lastname, firsname.