For starters, missions might seem like a distraction or a waste of time and resources but they’re very important. They help you learn the basics of the game and reward you with special points called Mission Credits.

Once you've completed all of George's missions, you need to reach level 5 to unlock new contracts from Duke. Duke offers daily missions that need to be completed within 3 days.

You can have maximum of 3 active missions. If you dont have a empty slot (less than 3 mission) at 4th day, then that days mission is unavaliable for you. So dont try to stock them unless you are in middle of faction war.

Completing Duke’s missions not only gives you promised rewards, but also increases your standing with him. The higher your standing, harder but also more rewarding his missions gets

All Duke's missions and tips about it can be found here: https://www.torn.com/forums.php#/p=threads&f=47&t=16401019&b=0&a=0

About Mission Credits: I strongly recommend saving your Mission Credits only for books and high stat weapon parts. I have been told they are extremely rare and only easy OR ONLY way to get them is collecting like 1000-2000 points to buy them from mission store.

If you're ever confused about a mission or need extra help, don't hesitate to ask for help in faction chat.

Good luck on missions and have a nice day!