2d Square Rotations

To do this I know you need to use trig functions, but I cant seem to get it right. Anyone know of some source code that does this properly?

I am having major problems getting this to work.

First of all, I sort of understand the general idea behind matrices, but I'm a little lost trying to implement them.

Second, I'm not sure this is even going to do what I want it to do.

Right now, the way my program is working, If you try and rotate the square, it just kinda disappears.

I want my program to
A: Display a square anywhere on the screen.
B: Be able to rotate that square as if a tack were placed in it's center.
C: Be able to change directions of the rotation whenever.