Well, you have to find the missions that correspond to your rank. Let's say you're Chuunin rank. To rank up, you need to do at least 3 C rank missions. To complete them, most Missions have a location attached to them and says where it should be completed. If a location isn't given, then its able to be done anywhere.
Once you decide which one you're gonna compete, let's say Planet of the Apes which requires battling 6 monkeys, you need to see if it's a purely RP based mission or a battle based one. Pure RP based ones are missions that don't require you to battle against anyone, just reach some goal via roleplaying. This can be escorting someone somewhere, or deciphering a murder, etc. Battle based missions typical require another RPer ( a Sensei ) to take the place of the NPC you fight or have a corresponding mission that allows someone else to compete while battling against you. In the case of the Planet of the Apes mission, you'd require someone to act as NPC to control the monkeys you fight against.
Once the goal and objective have been met, the mission is considered done and you can submit it when ranking up.
This is the right section; this forum can still be used for these type of questions.
Unrelated: your username doesn't show up for me on this phone, just a blank but my other phone shows it. Weird
