The index finger goes across the second fret. The top 3 middle strings can be hard to get at first. Think of it like this. Play an A chord on the second fret with your pinky, third and second finger leaving the index finger free. Then
slide the A chord up to the fourth fret. Lay your index finger across the second fret and you have a B barre chord. An easy way to do it is to just mini barre with your third finger across the 3 strings in the middle. Does that help.