If you NEED to use Runge-Kutta, then this is a homework assignment, and you are not telling us, because you know that we don't do your homework assignments here.
If it IS your homework assignment, then surely you can find pseudo-code to implement Runge-Kutta online? Start here:
If not homework, then you really don't need to use specifically Runge-Kutta, do you? And certainly, you don't need to write the code yourself. So start by reading the help docs for ODE45, looking carefully at the examples. Depending on the extent of the nonlinearity, that may suffice. If not, then you will need to spend some time actively researching methods to solve nonlinear ODE systems. But I'd bet this really is about your homework.