How to sign CA over apk generated keys and how to check our app is signed with following certificate

I'm junior android developer and a question is asked from me that don't know the answer ... Could someone help me and describes in the way I can understand or send links about that? question: when we want to release android app we have a key generated with app, how we can sign these keys by a CA (certificate authority) and how to check the app is signed with current certificate in code? I've searched in many sites but didn't get clear answer ,I'll be thankful if anyone can send source code to show me step by step procedure in code