I suggest you go through theGetting Started section of the documentation to familiarise yourself with Simulink. Also have a look at theSimulink tutorial section of the web site. There are various demos in the product as well, which you can find in the help browser underSimulink ->Demos.
For your particular problem, you will need the following blocks to compute du/dt as a function of u:
One you have computed du/dt, you can get u(t) using anIntegrator block, and close the loop by feeding the output of the integrator (i.e. u(t)) to the blocks above, that compute du/dt as a function of u(t).