Go up to the Apple menu and select "System Preferences..."

Click on the Time Machine icon in the window that appears....

Click on "Change Disk..."

On the screen that appears, choose "None" as an option and click on "Stop Backing Up"

You can close the System Preferences at this point.

Next, you need to delete two files. The first is found at the location Macintosh HD > Users > username > Library > Preferences > ByHost....

...and will be named "apple.com.TimeMachine.xxx.plist" (where "xxx" could be any number of characters)

The second file is located at Macintosh HD > Library > Preferences....

...and will be named "com.apple.TimeMachine.plist"

Next, reboot your Mac.

Once your Mac reboots, go into the Time Machine preferences again and click on "Choose Backup Disk"...

Choose the disk you want to use and click on "Use for Backup", then check the options to see the default settings.

Note that File Vault and Time machine are not the best of friends. Apple often recommends turning File Vault off if using Time machine unless you really need profile encryption. Time machine is not able to restore individual files if File Vault is active. Instead it has to restore the entire profile. Also, it seems that Time machine incorrectly assesses the space required for backup due to File Vault using a lot of temporary space.