The easiest way to get going is to check out many of the free or fee-based designs that exist on the internet. Here are some websites that you might want to check out:
You could also do a search to find more sites. In general, what you are looking for are *.stl files, which stands for Stereo Lithography. You can go to your favorite web site that offers 3D models (my favorite is www.thingiverse.com) and download the *.stl file of your choice. You can then view this file by using a free, downloadable *.stl viewer, such as www.freestlview.com or www.viewstl.com; the latter is the one that I use.
To get the part printed, there are several resources. One of the most popular is www.3dhubs.com. This website serves as a clearing house for all the local 3D printers in your area. Enter your ZIP code and you will be able to find a list of available printer resources, get a quote, pay for the part, and, in a couple of days, receive your printed part.
You will be asked several questions. The first concerns your desired material. I always use ABS, but nylon, PLA, and other materials might also be available. Next is the size of the finished part. Your *.stl file might be a different size than what you want. Give them the approximate size of the object that you want printed and they should be able to get close. Finally, they will want to know the resolution of the printed part, typically low, standard, or high.