How do I back up a database to a .bak file?
From a SQL prompt:
BACKUP DATABASE MyDatabase TO DISK='E:\MyDatabase.bak'
Go to Microsoft Server Management Studio and right click on your database name. Go to "Tasks" -> "Back Up..."
Then assign your properties, ensure that "Backup Type" is full.
Then at the right there is a button "Add" press that and set your filename, ensure that you place the extension .bak at the end of the file name.
Finally hit ok and wait for the backup to complete.
MSDN also has an article that explains this http://msdn.microsoft.com/en-us/library/ms187510.aspx
In ssms right click on the database, go to tasks, click back up. should be self explanatory from there.