Doddy makes a good point, you'll need a program to produce your drawings (CAD) and generate the G-code (CAM) up front.

Fusion 360 is becoming very popular as it's a free and very powerful 3D tool and does both CAD and CAM functions but is not very intuitive. I've yet to spend the time needed to master it though I've tried it out several times. Others may disagree regarding it's ease of use.

CamBam is a 2D drawing package and CAM generator which is not free but is well worth the one-off cost. All future upgrades are free, as are a wide range of very useful extra plug-ins available from a strong user base. CamBam is easy to learn and use.

Kit