Is there a program to do this?

Well. I recently started to use tweet attacks to follow and unfollow people ( free edition ) . I notice when i follow people ..some peope then tweet

" Shoutout to @mytwitteracc for following me ".

is that a automatic tweet? or do they just sweet that manually?

Both. I'm sure some people do it manually but you can also do it automagically. I use my own scripts but I know that some other off the shelf software does it too eg. I think you can do that with Twillow from memory.