Welcome to the forum and congratulations on your successful first build (there'll be others).
A fast idle can be a symptom of an air leak in the induction system. The most common place is the carburetor where it attaches to the intake manifold. You'll want to make sure you have an air tight seal at that point. The manifold gasket to the cylinder would be the next place to examine. As Saddletramp suggested, the idle speed screw should adjust the speed but if it still idles fast with it screwed out then an air leak should be your next area of inspection. There are other causes but let's start simple and go from there. Get back to us if you need further help.
ooops, a little slow on the keyboard. Joe is faster