and before you start saying “well im sure the british were better shots so im technically still right”, no, they were not. american farmers/civilians were used to hunting game and so were generally just as good at shooting as the british were. on top of that american long rifles developed for hunting had much higher accuracy than the standard british rifles of the time. the difference in soldier performance was purely military discipline and training to function as a unit/army - washingtons army had huge issues with desertion or breaking apart mid battle due to morale.
to round out the brown townogy, the rebels in star wars generally came from frontier planets afaik so would have had to face dangerous flora/fauna and would not have been nearly as untrained as you suggest. given that their top leadership did have experience fighting wars previously, its not too far fetched to suggest that they could fight effectively against the empire. they’d never win a full frontal fight, but they didnt need to to win. like most gigantic bureaucratic governments, the empire was slow to respond and had a lot of territory to defend while the rebels didnt.
this doesnt excuse the stuffty deus ex machina plot devices, just explains why its feasible for the rebels to win/not be immediately crushed upon forming.