$$\begin{bmatrix}

a_{11} & a_{12} & \cdots & a_{1n} \\

a_{21} & a_{22} & \cdots & a_{2n} \\

\vdots & \vdots & \ddots & \vdots \\

a_{n1} & a_{n2} & \cdots & a_{nn} \\

\end{bmatrix}$$

You can also use

$$\begin{Bmatrix} a & b \\ c & d \end{Bmatrix}$$

Or

$$\begin{pmatrix} a & b \\ c & d \end{pmatrix}$$