I use a 1/8″ upcut 2 flute mill the most, and then some .1mm engraving bits, then an 1/8″ downcut 2 flute. Occasionally I’ll use a 1/4″ mill.
If you just randomly looked at my router, it’s most likely to have the 1/8″ upcut or .1mm engraving bit set up.
What I found was helpful was to go to the local woodworking store (woodcraft) and talk to their people about end mills for 20 minutes. That gave me a lot of detail towards getting started.