I use free online cad programs like Tinkercad or Fusion (both products of Autodesk). YouTube University has been amazing in learning how things work. There are times I start from scratch and build an item. There are other times that I will I find an object in Thingiverse (free online makers community where people upload their designs so others can print or expand upon to suit their needs).
Fusion also has an amazing feature that allows users to pull items out of the McMaster-Carr catalog and edit/build on them in the cad program. I used this feature today while creating some upgrades to my 2” PVC fluid bed.
As far as getting a 3D printer. Just know they are not an appliance like a microwave. They have to be tinkered on, adjusted/bed leveled regularly, and stuff breaks that you have to fix. Again, YouTube University or online message boards to the rescue. The one best piece of advice I can give is buy one with a heated bed from a reputable company. I got my Anycubic I3 Mega from Amazon on sale after months of research