hi there, here's some random vertical bullet-hell-y games off the top of my head that i think are relatively beginner-oriented:

on steam:

bullet hell monday finale (free, upgrade-based, short missions, probably great for a total rookie)
devil blade reboot (may be a bit fast? but such a beautiful game!)
angel at dusk (same deal as BHMF but with a really unique style and not free)
mushihimesama (classic, good easy mode but may still be a bit fast, check a video)
graze counter gm
like dreamer (plenty of loadout options and extra features, very smooth game)
gunvein (may be a bit fast, but has very in-depth tutorials that focus on building skill)
dodonpachi resurrection
crimzon clover world explosion
zeroranger (fairly different from the other games here but a lot of genre rookies like it)

not on steam (sorry, i can't help you get your hands on these):

touhou 08 ~ imperishable night (tons of great features and stuff to do, amazing vibes)
espgaluda (one of the easier classic arcade bullet hells, slower bullets, gives the player a lot of options for dealing with things)

all these games except espgaluda have easy modes or something like that, so check out some videos and pick whatever you think looks cool and on your level. hope you find something you like!