how can i change sprites and add an enemy AI

Change sprites with SPCHR. However, adding an enemy AI is NOT a simple question. There is not any particular way to do it, so you need to think of what you want the enemy to do and then program it thusly. You can ask questions about something if you don't know how to do it, but adding an enemy AI is too broad a question.

i know how i want the enemy to so thats my start

So what do you want the ennemy to do?

look at my discussion pocket artillary