Reprap is a very popular Open Source 3D printer. The idea of this project is of a self replicating machine that can create copies of itself. So when you want to build your own 3D printer, the first thing is to get a set of 3D printed parts. It is a good idea to dive into the RepRap community and get to know people that can help you acquire them. You have the option to buy a complete DIY 3D printer set that includes all the parts, but this is of course less fun than assembling everything yourself.