How can I retrieve frequencies from a discrete time signal, lets say in matlab?

Let us generate two sinusoidal freq and add them togather and plot them. Now sample them and based on sampled data we are retrieving the frequecies contain in the signal. We have to use fourier transform (DFT) but how many points DFT we use? what is criteria. I know there is an example in matlab but I need something more illustrative.