lm2596 fixed 5 volts(usb) from 12v 10W solar panel
If you have the 'fixed 5v' version (LM2596-5.0) then you can connect the feedback pin to the output. 
But since it seems you actually have the 'adjustable' version (LM2596-ADJ) then you'll have to use the correct voltage divider between the output and feedback pins. See the instructions starting towards the bottom have of page 12 of the datasheet.
You'll get 5v out as long as your solar panel gives you at least 7v in.

The 12v input for 5v output shown in the datasheet is just an example. This IC will quite happily run off 40v and give you a 5v output - check the 'LM2956-5.0 Electrical Characteristics' table on page 3, or the LM2956-ADJ table on page 4.