jungling is one of the most intimidating roles to learn, there's alot of concepts that are unique to the role
luckily, the requirements of being a good jungler is most similar to the mid and support roles. if you roam alot to gank as mid, you already have a good understanding of when and how to gank. jungling and support are similar in that both should have great map awareness and have a good understanding of where the enemy is in the fog, such that you can navigate the jungle areas safely even without vision.
which jungler you should start learning with depends on your elo. if counter jungling is prevalent around your skill level, you want to start with a safe jungler such as warwick. if counter jungling doesn't seem to be a problem, then you can pick strong junglers such as amumu, fiddlesticks or maokai without fear of being counter jungled.
generally, it's best to start with a champion you're comfortable with in lane and transition into the jungle with that champion - so you don't have to learn the champion, and the jungle at the same time. you want to avoid skillshot heavy champions unless you're comfortable with skillshots (lee sin, cho), even if those junglers are tier 1.
clearing: things to consider when choosing a jungler are
1. clearing speed
2. sustain (how low do you go on the first clear?)
slow junglers are vulnerable to getting counter jungled
junglers that get too low after first clear are in danger of getting straight up ganked - start cloth armor if you find yourself getting too low in hp
when you first start jungling, i recommend using a fast clear high sustain jungler.
you still get to learn about ganking, what makes a successful gank, but you don't have to rely on successful ganks to transition into the mid-late game.