In order to shield bash you must have at least the initial "Energy Shield" (first skill on the left) and the initial "Dash" (fourth skill on the right) skills unlocked. This will cost a total of four skill points, so you can start going for these kills earlier in the game (or just come back to them later on).
In order to shield bash, you must active your "Energy Shield" with LT (left trigger) and "Dash" with LB (left bumper) . As long as you're facing the enemy when you bash into them, they should take damage. Weaker enemies take usually 2-3 shield bashes in order to die.
Hope this guide helped! If you have any questions or concerns, lemme know!