Schedule a tasks scheduler job with the backup command. In the simplest form it would be something like this:
BACKUP DATABASE [YourDbName] TO DISK = N'C:YourBackupFolder\YourBackupfilename'
More details here: http://technet.microsoft.com/en-us/library/ms187510.aspx
You can also schedule it using SQL Server Agent, and most people seem to prefer doing the backups using SQL Agent.