Assuming you're tuned to DGBE the usual fingering is 2210 so middle finger on 4th string 2nd fret, ring finger on 3rd string 2nd fret, and index finger on 2nd string 1st fret
Usually the middle finger. I don't play a baritone, but I assume that most ukes would be the same. I use my middle finger to make the Am on my concert and my soprano.
You should use your index finger on the first fret of the second string, middle finger on the second fret of the 4th string and ring finger on the second fret of the third string.
Remember: you count strings from the thinner to the thicker ones, on baris, E is the first string and D is the fourth.