Select

I have 2 tables.
The first one contains all information about the customers.
The second one just contains an username field.

SELECT  username, firstname, lastname, birthday from TableA
(15,000 rows)

SELECT  username
FROM  TableB
(2,000 rows)

What I would like to do is show all the information from TableA JUST for the usernames registered in tableB.
So I would have username, firstname, lastname and birthday just for the users registered in tableB

How can I do that.