League of Legends (LoL) Question: Starting JG

Remember to always go for tracker's knife jungle item. Placing wards in the enemy jungle is a good way to start, no matter the champion. If you can't gank as often at least try to protect your lanes against surprise ganks. Don't have to keep buying pink wards, just during objectives like drake or herald. I stopped buying the warrior and skirmisher a while ago and it has really helped me play much more clearly.

there isnt a perfect start. You must be that guy who doesnt know how for few games untill you grasp the basics. The only way is experience and doing mistakes to learn from them. id pick a less skill demanding champ, like WW, who also has great jungle sustain and gank potential. Or maybe Vi. Focus on learning good farm patterns(dont avoid become dependent on them) and generally try to not fall behind.
Dont try to help/gank all lanes. FOcus on one as a start(i wont go into details how to pick the right one) and feed from there; just try to get fed. Then play for objectives. Try to be there when its good time for drake, baron etc, but not without your team. You cant do much on your own. If your team doesnt support you, dont waste your time; even taking a jungle camp can be a better option.
Practice and learn on different gank approaches. I understand you already know a fair bit about the game, what champs does, or different ways of ganking, so you just need to manage to put this knowledge into practice.
PS: there is no way not being that guy who doesnt know, even if you do know, even if you do great(even if you been jungling for 5 years) you'll still get accused of not ganking certain lane, or being bad for whatever reason, or pretty much, for not doing what your teammates expect you to do(which is to babysit them- all of them). Just need to now let that get to you. Ignore such people, 90% of the time they are wrong and are not even worth the attention and are the bad ones, who fail the game

The most important thing about jungling is clearing camps as fast as possible while also finding time to pressure the enemy jungler (counter-jungling) and gank lanes. That's what you'll want to aim for when playing jungle.

So all this basically means that you're gonna have to find a balance between ganking and clearing camps in the jungle. If you farm for too long or constantly neglect a friendly lane that's pushed to its turret, your team will miss the chance to grab some early kills and snowball from there. You must always make a habit of checking the map to see if there's any available gank. Early mobility boots help a lot with roaming as well, while deep jungle wards can help both you and your team by revealing the enemy jungler's location (and his route).

In order to practice and "not be that guy", all you can really do is pick a jungler you find interesting and practice clearing camps in different routes on Practice Tool. The faster the better. This will not do, however. As I mentioned earlier, jungling isn't all about clearing camps. You need to play normals in order to fully grasp the role. Don't mind if other people start flaming or accusing you for losing their lane, no one ever got banned by trying new things or perfecting their roles.

Finally, if you check the most played champions from each meta, you'll realize that most of them have some kind of dash or speed boost. That's also why champions like Lee Sin (fast clear, 2 dashes, high early game damage) have never really fallen out of any meta. There's always exceptions that compensate for their lack of gap closers by offering faster clear or a lot of hard CC ( Amumu for example). You can keep this in mind while choosing a champion to play.

Hard to say without knowing what knowledge you have. If you're already aware of how the camps work and standard jungle routes, you might just need to practice.

Watching high elo streamers or maybe pro games can give you ideas for good pathing for various champions. I recommend choosing one champion ( Warwick is a good starter but you can go with someone else if you like) and studying/practicing them a lot. Once you learn one jungle champion, it's easier to pick up others.

If there's anything specific that's confusing to you, let me know and I'd be happy to help.

Watch people play jungle, and read the guides.
Each jungle champion has its own clear path, and its own way to win. Lee sin is all about setting up his team with his early ganks and counter invades. Kha'zix is more for himself, getting fed and being able to carry the team by deleting all damage dealers on the other team. So you should look at a jungle you like the look of, perhaps youtube some gameplay or a guide so show you how to use them, then read a guide on here to understand more about the champion, the good guides on here with usually tell you the clear paths for that champion and what item works best.
There is always the practice tool to help you with your jungle clear times and how to counter invade.
As for champions starting off, Master Yi is easy to learn and would be a good start but depending on your play style you can go from there. My advice, if you like assassins, Kha'zix is in a good place right now (Im a little biased, he's my main) but if you like tanks, Lee or Vi are good picks but keep in mind their skill ceiling are much higher than Master Yi so do your research.