Making a semi-automatic pistol using a 3D printer and parts purchased on the net couldn't be easier. If I can do it, so can you.
You will need:
1) A 3D printer. Ideally, a big one capable of printing large objects. If not, you can print the receiver in sections and glue the pieces together with Krazy glue, super glue, or in the case of ABS, acetone weld. This is what I did the first time around.
2) Strong filament. ABS, HIPS, nylon, etc. are top choices, but believe it or not PLA works. It just doesn't last.
3) A CAD file for a Ruger 10/22 or Ruger Charger receiver. There are plenty floating around. The first pistol I made (the neon green one) used a CAD file designed by Todd Aho of Taho Designs. It was designed for CNC milling, but it worked okay with some Dremel and sandpaper fitting. The one I designed with iron (plastic) fixed sights prints ready to go. Unfortunately I cannot release it because of ITAR. Find something similar that fits with what kind of gun you're after, or design your own. Like I said, there are plenty of files floating around in the aether.
4) A full set of parts for the Ruger Charger or 10/22 (parts are compatible between the two weapons). This will include:
Get a threaded barrel if you can. This would be a great gun to use with a 3D printed silencer (all NFA rules apply - be sure to fill out a Form 1 with $200 tax stamp payable to the ATF before printing any silencers).
Ascertain that it's legal in your area to build the sort of firearm you want.