Missions are pretty easy to do Some Require You to Get pokemon others require you to get items.

To Complete missions you have to do a simple RP like " Gerry goes and gives the pokeball to the kid that requested it " Or something.

Th Rewards Are Different Depending on the type of mission
some Reawrds Are:
Eggs
Pokegold
Items
Rare Items
Recipes
And much more.