First off, please try to stick to the guidelines for posting questions to this site. See this answer for help in that area:
While it's a little bit unclear what you are asking for, changing the multiply to a dot multiply in the line y=(A+ym).*sin(2*pi*fsig*t) got rid of the errors that I was seeing and at least produced some results. See below:
f_s = 200;
t = .1;
N = 2000;
delta_t=1/f_s;
start=0;
step=t;
stop=(N-1)*delta_t;
t=start:step:stop;
A=input('enter amplitude of the wave');
fmod=input('enter modulation frequency');
fsig=input('enter signal frequency');
ym=A*sin(2*pi*fmod*t);
ys=sin(2*pi*fsig*t);
y=(A+ym).*sin(2*pi*fsig*t); % I changed the * in this line to a .*
% I also removed the subplot command because it's not needed
plot(t,y);
Is this what you were looking for?
