What jungler should i train with, and some advices

So, i was a toplane main, and i want to play more jungle, but i don't know what champion should i start with. I need jungle that will help me to easly gank, don't need to have perfect jungle clear, but still not too bad. And i need some advices to: When should i gank and when should i leave, when to push with my laner, when should i take cs after 1 for 1 trade, when to counter jungle, and also when should i do dragon, and when should i leave it.
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