and what i get in NSolve is supposedly be the steady state, but i don't know how to find this 90% and my profesor told me that i could try do this with DSolve, but something doesn't work. i hope that someone will be able to help me with this.
Your question is difficult to answer, since you show only a part of your problem. It seems that you should have a differential equation describing the process. Otherwise, why z=z(t)? In this case, however, in order to help you one needs to see this equation.
Generally, if you have the ordinary differential equation, you may try DSolve, or, if your equation is independent of parameters, try NDSolve. You will find comprehensive examples at Menu/Help/DSolve and Menu/Help/NDSolve.
If you have equation depending upon one or two parameters, you may wrap your NDSolve statement by Manipulate or use ParametricNDSolve instead. I recommend that you try the both of them and then choose. Have a look at the corresponding places in the Menu/Help.
If your equation is a partial, Mathematica may only help in few special cases. Have a look into Menu/NDSolve: the heat and wave equation in one dimension in the end of the Section "Examples". If you write down your differential equation, one may give a more precise advice. Nobody will solve it for you here, of course, but you may await to receive a useful hint of how to use Mathematica for that.