Get lots of songs and delete every difficulty except the hardest (and maybe the second hardest so you can use mods on them) in every song you download, so get rid of every easy, normal, or sub 4 star map. The reason for this is usually people tend to put more effort in the hardest difficulty when mapping, there are some songs out there where their hardest difficulties are pretty good for beginner players.
Next Sort them in any fashion you feel comfortable with developing: bpm, length, star rating, etc. Practice them in various orders and try to pass with A/S rank on every one of them. If you get a C play something slightly easier, if you get a B retry until you get an A or S. Getting D ranks are a clear indicator the map is way out of your league. After a while go back and try to beat your scores with higher accuracy or with mods retaining the same accuracy or higher in fewer plays, doing so will develop consistency. If you have a 98 to 99% on a map don't settle for <96% mod play unless it's obviously reaaaally hard/rewarding rank wise. Keep at it until you can get at the very least a 97% and just come back to it later.
Aim usually requires a comfortable consistently playable setup so you can develop muscle memory quickly. With mice you disable any acceleration or angle snapping. On tablets map it to a size that is simple for you to reach all four corners without straining yourself (keep in mind if you're playing on a 16:9 or 16:10 the playfield is actually 4:3 so you have room to map it larger, if you don't understand what I'm saying about this then I can clarify it in another post). Aside from the setup it just takes practicing a variety of maps with lots of different patterns. Mostly you want to focus on each note and try to stop at the very center of it as quickly as you can.
Just don't get stuck on focusing aim training like a lot of beginner players tend to do, stream training is just as important. For this it requires you to play to a tempo, usually streams are mapped as 1/4th of the bpm. This requires you to hit 1/2 notes with each finger in succession. You can train your fingers individually by playing maps with lots of half notes. When getting to actually hit streams you want to listen to the sounds in the music, I like to focus on one finger to make sure that I hit the half notes in the streams just right and then I follow through with my other finger. Streams tend to be the hardest thing about osu! in my opinion because there are such physical limitations as well.
So going back to my first paragraph, play 160bpm maps or 4 star maps, whatever you feel comfortable with and go up from there. If you fall back on easy maps and play them all the time you really wont get any where with this game.