In rom form with modern emulator magic there isn’t as serious of a learning curve as on original NES although I would still recommend MegaMan 2 as where to start.
MegaMan I is difficult, extremely so when you had to do the whole thing in one play through. Megaman 2 added both a password system and health refill as well as a larger selection of secondary weapons.
The basic concept is that you can pick the stages to play in any order, as you defeat a robot boss you gain their power as a secondary weapon. While I think you can go through the whole thing with the blaster, each boss falls extremely fast to one of the other bosses’s powers so there is a best order the levels should be done in.
It’s also classic 8-bit music,