Now When it comes down to missions this is how they work. There will be basic missions laid out everyday for people to do to earn zen. Without zen you will not get very far here because you will run out of items to use as well as a very small amount of jutsus. You are limited to 1 mission a day. some might take even longer than a week worth of posting for an example an S-Rank mission.
D-Rank Mission- you earn 300 Zen, needs to be Genin or Higher to do this mission
C-Rank Mission- You earn 600 Zen needs to be Genin or Higher to do this mission
B-Rank Mission- You earn 1200 Zen needs to be Chuunin or higher to do this mission
A-Rank Mission- You earn 3600 Zen needs to be Jounin or higher to do this mission
S-Rank Mission- You earn 10800 Zen needs to be Jounin or higher to do this mission