That's ok.... Backup's are essential, and easy.
To partition the new external HD, when you plug it in, go to disk utility and select the Ext HD in the left column.
Click the partition tab and select under volume scheme, 2 partitions.
Click on one and format as Mac Os Extended (journaled) and name it Clone.
Click on one and format as Mac Os Extended (journaled) and name it Time Machine.
It will look something like this:
Come back for instruction on time machine or the clone or both when you are ready