Auto Reply SMS (Android Programming)

I am relatively new to Android Programming world and would appreciate your help! I have managed to write a code for one sample activity, which upon launching my app scans through inbox/sent messages folder and can filter out messages from their content and/or sender number. I want to design an app which keeps on listening, in real-time, for all the incoming messages and reply "Success" (OR reply some text) upon some filters on SMS content. I, honestly, have no idea about how would I proceed from here. Can you please outline general design and APIs I need to use? Thanks In Advance!!