I am using real data of wind, which is a random one to get the power spectral density of these values.
How can I do to get this PSD in matlab, and Can I get an analytical expression for this psd.
The standard method is to use Welch's algorithm, which you can access via the
pwelch command in Matlab. It is pretty well documented.
To get an analytical expression you'll have to use some kind of curve fitting. The difficulty of doing this will depend on the particulars of the PSD estimate (it could be nearly constant, for example, or have many maxima and minima, or somewhere in between). Matlab has several methods to calculate curve fittings.