Finding Null space of matrix when mI know how to find null space of a matrix, by following certain procedure like this.
But how do I find null space for this matrix?
[ 1 0 0
  0 1 0 ]

Here, the number of rows < number of columns (m<n). I'm not able to represent pivot variables in terms of free variables.

You want to solve the linear system
$$
\begin{array}{l l l}
1 \cdot x_1 + 0 \cdot x_2 + 0 \cdot x_3 & = & 0 \\
0 \cdot x_1 + 1 \cdot x_2 + 0 \cdot x_3 & = & 0.
\end{array}
$$
Therefore $x_2 = x_1 = 0$. Those are the pivot variables. The free variable $x_3$ has no influence whatsoever on the left hand side, so $x_3$ is arbitrary. The solutions space is thus
$$
S := \left \lbrace (0, 0, x_3)^\top \in \mathbb{R}^3: x_3 \in \mathbb{R} \right \rbrace.
$$
