Let's see , when I started to learn jungle in season 5 around the time of Ekko's Release, I picked simple champions with a lot of safety like Sion, Evelynn and Vi before I did something like Kayle and Riven. I watched a lot of youtubers play in the jungle (I think at the time SolWolf and Redmercy were my current favorites) so I had a small idea of what the jungle was like.
After I got a feel for the area itself, I began to work on things as they came up like what to do on an invade, when to invade (which I can't do still, lol), and when's the best time to gank. I would focus on improving my clears before I did anything else to the point where, as Vi, I was confident enough to clear krugs/gromp with enough health to gank bot or top lane and get an easy flash or two.
Pretty much pick "safe champions" I would say; nothing with incredibly high skill ceilings or multiple passives to work on. Watch how other people play the lane to at the very least get a feel for what you should be doing. Work on things specific for that role like top lane TP ganks, Mid lane Roams, jungle ganks, or ADC farming.
Fail as hard as you can because that's the best way to learn. I didn't learn roaming patterns as Lux by doing it amazing the first time, I studied my opponents, checked the map, and took risks when I felt I could because I went through a laundry list of what I could/should be doing.