what i'd do is to play one 9k map over and over until i nomiss it. this is usually discouraged by the mania community which will tell you you're just building muscle memory for one map, which is true on higher level maps, but as an absolute beginner it's a good strat becuase it makes it's an easy way of training your fingers to be able to actually hit the notes. it might feel like you're not improving your scores at all at first, but just keep playing and soon you'll improve your skill for the map quickly. when you get a full combo, move on to other 9k maps and keep improving your skills on them.
as for maps, there's very few ranked 9k maps (about 60 as of writing) so to increase the size of your mappool you'd need to mostly download unranked maps.
i have a handful i can share with you, so dm me and ill send some of them to you once i get home.