The number of clusters is the only parameter you need to give to fitgmdist and mean and covariance matrix will be calculated based on the data that is being processed. The documentation you mentioned does slightly advance analysis around the covariance matrix being full or diagonalized. If you need analysis on you fitting, then I think following the documentation step-by-step will help.
Otherwise, if you only want to do the clustering then these two documentations will be quick to understand.
For selecting the number of clusters, you can plot the data to see how many discrete clusters visually you can depict from the data. You can change your initial estimate by +-1 to see which one gives least log-likelihood.
Hope this helps!