You should most likely focus on learning how to model or do the cad work correctly. That is really the end of your duty as a buyer, because the shop you hire must and will use their own savvy to produce and program the part as they see fit. Considering your inexperience, it will not do you much good to produce machining processes that are not suitable for use in the real world.
Now OneCNC can give you some insight into the complexities of programming and you will appreciate what must be done to produce the part. But without hands on experience of programming and then cutting it yourself, you just will not learn enough.
But to get your feet wet with solid modelling in cad, if I were you I would check out some of the student versions of the popular cadcam systems. Maybe Solidworks (because it is very popular in industry and is cool to boot), or Rhino, just to name a couple of popular chocies. OneCNC also has a Solid creator version available without any CAM, but I don't think there is a student version.