I’ve never used a 3d printer before but want to get one. I have a bit experience in blender but not too much. My question is: How do you model for a 3d print? For example, if I want to print a hollow cylinder, I go into blender, create cylinder and delete the side faces. If I print this, the walls will be pretty thin. Do I have to make them bigger manually? and if I do so (extrude and scale) does my slicer (cura) automatically fill in the solid part?
Start off with Thingiverse or similar. I recommend something like: https://www.thingiverse.com/thing:3553160 There are a lot of models there - those are .STL that you “slice” and send to your printer. There is this: https://www.thingiverse.com/thing:2187167 which is CAD models with the working left in - OpenSCAD in this case. You load it up, generate a .STL and then pass that to the slicer.
I have a large plastic cabinet in my garden for storing a lawn mower etc. The hinges died years ago. I have printed new ones. At least 20kg of plastic waste has been avoided being dumped a lot longer. I am well aware (now) that I should not have bought the bloody thing in the first place!
If you go the OpenSCAD route, you might like this: https://github.com/JustinSDK/dotSCAD - the author sadly passed away recently but his work is legendary in my opinion.
Once you get printing sorted out, then move on to your own stuff. … or not - give it a go! I have a large bag of very strangely shaped PETG experiments that went badly wrong and need recycling.