ZBrushCentral

I wan to use a tool I created and need a geometric to start the drawing.

How do I bring a geometric into the window so I can start drawing from it?

This is something that I really really find interesting but still confused that how did you get that in the window.
Well, for simply drawing the sketch you can use Paint application as well. I do not know whether its going to be work properly or not.

I think these tutorials can help you in this matter:
http://www.zbrushguides.com/painting-geometry-resolution-zbrush/
http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/subdivision-levels/hd-geometry/

Also, if you need geometry homework help, in general, you can find some useful info on such resources or find basic courses on WolframAlpha.