Hi Tim,
Welcome to the forum.
Using Mach3 is, I think, the easy part of the CNC process. Initially, you need to decide on the CAD/CAM software you are going to use to design your parts and then create the necessary Gcode for Mach3 to use. There is a lot of free software available (I started out using G-Simple
http://www.gsimple.eu/ ) but probably the best (reasonably priced) and easiest to use, paid for, software is the Vectric range
http://www.vectric.com/. I currently use Aspire but it took a while to get together the necessary funds to upgrade form V-Carve Pro.
Once your CAD/CAM is sorted then loading the Gcode into Mach3 and setting the start datum point etc. is relatively simple (the Mach manual, I think, explains all
http://www.machsupport.com/help-learning/product-manuals/ ).
That’s about it really.
If you have any specific questions then please post them as a New Topic in the General section where they will receive the maximum coverage and hopefully a speedy reply.
Tweakie.