I would like to run a game and maybe a campaign of MG for only one player. Is there anything different that I should do to get it to work smoothly?
Should I use one of the scenarios from the book or would it be best to have him make his own character?
Play off of Friend and Enemy, and don’t use too many Wilderness hazards (they have pretty high Obs, for the most part). When using Animal hazards, find animals that can cause a problem but aren’t out to eat the mouse – crow or raven instead of an owl or hawk, for instance. Watch their Natures. If they’re too high or include “Predator,” every Animal hazard will be a problem.
Also, remember to use conditions instead of twists if you don’t want them to constantly come up against situations in which they’ll fail. You also may want to start them off with 1 Fate and 2 Persona instead of 1 and 1.
Escort and rescue missions can be good because mice are involved and can lend helping die(ce).
That’s up to you and the other player, really. Probably best to talk about the sorts of missions you’re both interested in and then the player makes a character to suit. You may want to play the first season in two or three places and have most events occur in those places (to cut down on Wilderness hazards due to traveling all the time).
You could also NPC a tenderpaw and have the player be the mentor. Patrol Guard is a pretty good rank for a solo game, I think.
i decided to make a character the first time and he turned out AMAZING. lucky me. his name is Natch and he is 5 nature, fighter 3, hunter 3, scout 2, wethr wtchr 2, armorer 3, orator 1, persuader 1, militarist 1. 7 resources, 7 circles… is this too good? did i do something wrong?
You can’t have 1s in a skill or wise so something went wrong. 
What rank did you go with to get Circles and Resources both at 7? Those seem quite high.