4th order Runge Kutta for a system of ODEs

I have a matlab code that i found in my textbook to implement 4th order Runge Kutta for a system of ODEs but I am know sure how to input my dydt or my varargin. here is what i have...

C = -.95121212 b = 1.479843791 k = 67.1170859 m=1 f(t)=sin(0.5t)

State Space X1 = u (X1)dot = u_dot = X2 X2 = u_dot (X2)dot = u_doubledot = f(t)-[(k*X1^b)/m]-[(c*X2)/m]

my output should be a bunch of y-values that i can graph with my t-values. Any help would be greatly appreciated

function [ tp,yp ] = rk4sys( dydt,tspan,y0,h,varargin )

%4th order Runge Kutta for a system of ODE's

% Detailed explanation goes here

n=length(tspan);

ti=tspan(1);

tf=tspan(n);

if n== 2

    t=(ti:h:tf);
    n=length(t);
    if t(n)<tf
        t(n+1)=tf;
        n=n+1;
    end
else t=tspan;
end

tt=ti;

y(1,:)=y0;

np=1;

tp(np)=tt;

yp(np,:)=y(1,:);

i=1;

    while(1)
    tend=t(np+1);
    hh=t(np+1)-t(np);
        while(1)
        if hh>h,hh=h;
        end
        while(1)
            if tt+hh>tend,hh=tend-tt;
            end
        k1=dydt(tt,y(i,:),varargin{:})';
        ymid=y(1,:)+k1.*hh./2;
        k2=dydt(tt+hh/2,ymid,varargin{:})';
        ymid=y(i,:)+k2*hh/2;
        k3=dydt(tt+hh/2,ymid,varargin{:});
        yend=y(i,:);
        k4=dydt(tt+hh,yend,varargin{:})';
        phi=(k1+2*(k2+k3)+k4)/6;
        y(i+1,:)=y(i,:)+phi*hh;
        tt=tt+hh;
        i=i+1;
    if tt>=tend,break,end
        end
        np=np+1;
        tp(np)=tt;
        yp(np,:)=y(i,:);
        if tt>=tf,break,end

        end
plot(tp,yp)
    end