My advice would be to complete Koffie's client orders as fast as possible to simply unlock all of the missions and just take other client orders as you go since you can hold 20 at a time... you'll most likely complete a majority of them as you are completing other ones that are more progression based, go in a party always if you can, and if you can't then bring NPCs... they make more of a difference than you would think.
Oh, and always milk the hell out of the Emergency City Mission when it is available.
