What to print out first for a beginner

hi!
I would start from the small and fast-print model just to see printer is working. Usually, it's one of the inbuilt models like Benchy.
Then I would go to a model like this https://www.cgtrader.com/3d-print-models/various/test-models/3d-printer-test to see what my printer is capable of.
After this print, I would do the calibration (with Bambu Studio software) and reprint the previous print to see the difference.
Now, I would start searching for interesting models and start printing them by size, from smallest to much bigger. Just to be sure that I won't waste filament if something goes wrong.
In a month or two, you will be surprised how much do you already know and how easy it is to slice models in slicer by you own.

In domestic 3d printing, mainly fdm printers like most people use, I say 1.5mm is a good minimum thickness, sometimes less can work but stick to 1.5mm as I have learnt the hard way lol. Resin printers can go way lower I think something like 0.35mm or less I believe for a minimum thickness of walls.

Watch tutorials often of the specific printer you have and the slicer it comes with and then put that into practice and you'll see improvement as you go along, just have fun and the learning comes in quickly and becomes natural muscle memory.