This all depends on what you want to do with your CNC router. Each type of work has more than one software title that will work for you. It is important to understand what it is you want to do and what machining methods you will need in order to produce your work. The reason being that when it comes to software, there is a significant difference in both features and price on the multiple software titles out there. Depending on what type of work you are doing, you may be able to get a simple low or mid-range title that will do everything you need. The dependencies are mainly dimension, type of worktable (normal or PTP) , and loading & uploading system. Rapidtech will offer you very detailed advice while you tell us your situations.

Can I use AutoCAD?
 Yes, AutoCAD drawings can be imported into most software design packages as a dxf. This makes it simple if your shop is already using CAD drawings. This will keep you from having to redo all the work you already have invested time into. WinCNC, the motion control software, can also import a dxf and run directly from your CAD file. This is not recommended as you will have to know the tool offsets. It is always better to bring these drawings into your CAD/CAM software to produce the code. This gives you more control as well as a visual aid.