What is the first significant figure of any number?

The first significant figure is the first number that is not 0.

For 100, the first significant figure is 1, for 0.06, the first significant figure is 6, and for 0.000345, the first significant figure is 3, for example

The first significant figure is the first digit that is NOT zero and then the Zero start getting counted.

For example 0.005006

First Significant Figure is 5 and the second is the zero after 5 and the 3rd is 0 and the 4th is 6

The first significant figure of any number is the first digit that is not zero. So in 0.023 the number 2 is the first significant figure. After the first non-zero digit any further zeros in a number are classed as significant.