I deleted everything on my Mac by accident including the startup disk. What can I do to reinstall my startup disk?

▷ Try these helpful steps. 

The startup disk is full and I would like to use my second disk to start up instead as it has a lot of storage that is available. When I go to the System startup disk screen, it only shows the smaller full disk. How can I add the larger disk to be my startup disk?

Thank you

Joan