For 3d printing you need 2 things: technology and skills.
Technology is easy to obtain: all you need is cash. There are entry-level printers starting around 150 dollars and professional high-end printers costing hundreds of thousands of dollars. For hobbyists there are mainly 2 types of printers:
- FDM: Fused Deposition Modelling where the printer creates objects by laying very thin molten layers of plastic material on top of each other. These printers can produce large items upto 20x20x20 inch but will always show the plastic layers they are made of.
- Resin: Individual layers of liqued resin are cured by a UV light source. A LCD screen is used to allow/block UV light for each individual layer (pixel on = no light/cure). Maximum size of printed objects is typically a lot smaller than from FDM printers. however printing layers can be as thin as 20 micron (0,02mm) which results in invisible layer lines.
For either type you will also need to learn up skills on how to operate these printers, but there are many sources available for help (ie Reddit, Facebook etc)
Skills are a lot harder to obtain. 3d printing starts with a digital 3d model, which is then 'sliced' into individual layers before sending it to the printer. Creating a digital 3d model requires the skill to design a 3d model in a 3d design software (= another piece of technology!). Not only do you need to learn how to operate the software, but you also need to learn how to view/analyse an object and recreate that in the software. That can be quite a steep learning path.
I've started building up my design skills about 6 years ago and using Shapeways to print my first models like:
http://www.sixtharmygroup.com/forums/showthread.php?t=21814
First by using some free 3d design software before purchasing my own software from Cubify Design. Nowadays Fusion360 would be my choice. It is a professional 3d design software, which is free for hobbyists.
You can try to search the web for free models, but I've found those models always lacking in details and accuracy. Most 3d models available are intended to be used in 3d video games and not for 3d printing. Reworking a 3d video model into 3d printing might be more difficult and time consuming that designing it yourself from scratch.
My recommendation: if you want to start 3d printing, start with obtaining the skill on how to design a digital 3d model. Start with something small & simple and use Fusion360 to design it, and i.e. Shapeways to print it for you. As your skill level grows you can start designing more complex and larger items. As time passes and you build up your skill, the quality, price and ease-of-use of the technology will develop in your favor and there will come a time that you will want to buy your own 3d printer.
Just be aware that -though very rewarding!- learning 3d design & printing is alike a marathon, it's not a sprint and you'll probably encounter some failures on the way.