There are multiple way to do this, which you'll learn in just about every tutorial for Matlab. You can try the Onramp course offered (for free) by Mathworks, on any other you can find with your favorite internet search engine.
For your problem, the easiest might be to use an anonymous function:
m = @(t) 0.001*cos(0.005*pi*t+pi/4);
You can plot this with plot if you generate values for t, or you can let fplot do that for you:
fplot(m,[0 1000])
