You would want to apply scrolling values to all the objects on the screen when the player moves closer to the edge of the screen.
Some pseudo code as an example to show you how to scroll a background when you walk left and right.
if rightkey() = 1
if playerX < 600
inc playerX
else
inc backgroundX
endif
endif
if leftkey() = 1
if playerX > 200
dec playerX
else
dec backgroundX
endif
endif
This will check to see if the player's position is greater than 600, if it is then the X background scroll will increase, if the player's position is less than 200 then the X background scroll will decrease, otherwise the player's position will change.
You would then subtract the backgroundX scroll value to all the objects/enemies in your level when you draw them.