In ukulele tab the finger you use to pluck the note is indicated by a letter in italics above the tab. They look like this:
Each letter corresponds to a finger on your picking hand:
t or p = Thumb
i = Index Finger
m = Middle Finger
a = Ring Finger
So when a note has an i above it you pluck that string with your index finger.
You won’t always find the picking notation. Sometimes you’ll just see it at the beginning of the piece and you just carry on that pattern all the way through. Sometimes you’ll just get the fingering for tricky sections.
The names are shortened from: pulgar, indice, medius, annular.