You can use other people's 'designs' from the likes of Thingiverse. - A website where people share their designs..
You can draw your own items. - You need a 3D CAD'drawing package for this.. There is a learning-curve, and a little talent helps.
You then use a piece of software called a Slicer.. This (as the name implies) creates the information for each 'layer' you are going to print.
You then need the printer. - Either one which uses filament, or a Resin printer, that makes items within a bath of liquid resin.
Shapeways, is another website mainly (but not exclusively) where people can post their designs. - You can 'buy' a printed item.. You select the item, then decide what material you want it printed in. This affects the quality of the printed item.
It used to be that Shapeways had 'better' priunters, than those you may have at home, but this is no longer necessarily the case.
Their service does mean that you can obtain 3D printed items, without having your own printer. BUT what you get will depend on how good the original drawing/file is. What it is printed on (which printer) and what material it is printed in..
PhilP.