I'd like to sound a note of caution. It's very easy for someone without machining experience to take a 3D cad program and design a part that is almost impossible to manufacture, or will cost much more to manufacture than a functionally equivalent part that is designed by someone who has some insight into the machining process.
Say you have a pocket in a part (pocket in this sense is an arbitrary shape hole in a part that only goes part way through the part). Say you decide that it wants a 1/4" wide 45º chamfer between the walls and the floor of the pocket. That puts you into the territory of needing a 5 axis machine to do the machining rather than a cheaper 3D machine, or requires a lot of setup on a 3D machine, or requires a non-standard cutter. Whichever way you machine it, it costs more than a simple pocket with straight sides that meet the bottom at 90º. If you need a pocket like that for functional reasons, fine you bite the bullet and accept the cost. But a naïve design might include such a pocket without considering the manufacturability of it. Hopefully you can see where this goes for all sorts of other features, for knowing how to design a part so that it can be held in the machine while machining and so on.
Before you design parts and kick them off for someone else to manufacture you can save yourself potentially a lot of grief by learning a bit about the machining processes. You don't have to become an expert, just learn enough to be able to give a rough answer to 'how would you make this on a mill/lathe?'. Like, is this feature milled, or drilled, or drilled and reamed, or bored, or produced with a slitting saw, or turned. And to have an answer to "How do you machine all the features I want on this part while still being able to hold it (usually in a vice) to machine it?".
You can get a lot of this just from watching some basic machining videos on You Tube. There are worse starting places than the basic milling and lathe courses put together on the 'blondihacks' channel on You Tube. If you want to have a laugh and also watch some machining 'ThisOldTony' on You Tube is good value, but not as deliberately educational as blondihacks. Somewhere on You Tube is a series of videos on machining that MIT put together for students who needed to use the workshops to make parts but didn't have any machining experience - search for "MIT Machine Shop Videos".