Matlab AWGN() function
The power of a normal distribuition (so the power of your white gaussian noise) is defined as variance ( \$\sigma_n^2\$) of the distribuition.

On MATLAB you can generate normally distributed random numbers using the function randn (see: https://www.mathworks.com/help/matlab/ref/randn.html ).
Before you generate your noise it's necessary to know what's the power of your signal, in that way you know what variance you noise has to have.