I am learning SQL so go easy!

I have a couple of tables. The common column between them is a 'user\_id' column. Table 1 has a 'user\_name' column. I want to get all the user names from this column in Table 1 where the user\_IDs match and update a column in Table 2 named 'domain\_info' with: ('domain name\','user\_name').  

'domain name\' will be text string and 
'user\_name' will be from Table 1

All help is much appreciated
Cheers