CAD is relatively straight forward once you get past the initial learning curve but getting there can be tough if you have no background in it. I have tons of experience in solidworks and pro-e (effectively the same thing) but nothing in 123D so I can't really help with specific tips. Best bet would be to look up some basic tutorials and start with those. It's boring as hell to model stuff that you don't care about while you learn but learning by doing in CAD is very effective at least until you have the fundamentals down.

There are places you can go to have digital files made for you. Here would be one of them for instance. Depending on the complexity of what you're trying to design, your ability to provide specifics on the design (hand sketches, etc.), and the person you're dealing with, your costs to have someone else draw it for you will vary widely. As a for instance, something simple like your link you could probably find someone to do it for free, especially if you're able to provide a basic sketch with some dimensions. Something more complicated like a 3d printer on the other hand, could cost you thousands to have professionally engineered. If I think of the time I have put into my printer design so far and calculate what that time cost me based on my salary I've probably "spent" 1-1.5k on mine so far and I'm still a long way from done. That's not meant to scare you off, just to give you an idea of cost spread and the skill set behind proficient use of CAD software.

If you have the desire to do it I would say go the tutorial route and learn to do it yourself. Not only will you have gained a useful and potentially valuable skill in the process but you'll have the ability to design more complex stuff later once you do get your printer and start coming up with more ideas.