How will I add a save game feature. Please help!

Well , i think you should declare some Constant Varialbles probably Integer to keep track of user state, once done , you should save that variable into a sharedprefrences object (please try google to have a better understanding of permanent data storage in android ) the next step is to retrive the variable value when user reopen the app .

in short : declare , save , on restart check

Let's make it easy. A lot of games just have you restart the zone you were in when you quit. So now you just need zone info and player stats. You should write these to disk fairly often. There phone might dies. They might get mad and just turn it off. You get the idea. When the games starts up, initialize everything before they can play. You might have bugs you did not know about. It should be an easy process unless you have specific game events that only happen to a player once, then you need those recorded :D

If they turn of the game properly, it should be extremely easy to stop the game loop and record as needed.