Hi Mona7150,
You will define the modes number, e.g how many modes are simulated, but what will define the propagating mode (TE, TM or TEM) will be your geometric structure. If your structure is a hollow waveguide, you will not see TEM modes, because the boundary conditions do not allow. Otherwise, if your structure is a coaxial waveguide or a two or more wire structure, then TEM mode will be the first propagating mode. In CST Help you can see some examples could be useful.
Best regards.