There's no need of initialize the variables as globals, so remove that line.
Also, the ode call needs to be outside the function. And pass the other parameters as inputs to the ODE function -
m=2;
k=2000;
c=10;
w_f=12;
F0=5;
dt=0.005;
t=0:dt:2;
y0=[0 0.5];
[tsol,ysol]=ode45(@(t,y) testode3_force(t,y,c,w_f,m,k,F0),t,y0);
damping_ratio=c/(2*sqrt(k*m));
plot(t,ysol(:,2))
figure
plot(t,ysol(:,1))
%ODE function
function dy=testode3_force(t,y,c,w_f,m,k,F0)
f1=F0*sin(2*pi*w_f*t);
dy=[f1/m-k*y(2)/m-c*y(1)/m;y(1)];
end
