how to Build a referral and earning system

[SKYLINE TV]

i am currently working on a system, when  a user register and do a particular task  they will be credited with a certain amount, also the app will have a referral system every user will have a unique referral id when you invite somebody with your code when there do a task you will also earn a certain amount as a referral commission 

i have already design my data base  but i don't know where to start from here, if any one here has build a similar app should please share they code with me, 

if I didn't deliver this job my work will be at risk 

thanks