Like Sean mentioned you can use First[Position[...]] or FirstPosition[...] to get the first position of what you look for.
To get element from result, you could do:
p = FirstPosition[{a, b, a, a, b, c, b}, b]
Extract[{a, b, a, a, b, c, b}, p]
or
{a, b, a, a, b, c, b}[[First[p]]]
