Help a noob jg?

Amumu is good, and my own go-to jungler for ranked as well (not that I play much ranked), but give Warwick a try. People assume he's bad because you can cancel his only hard CC but that doesn't mean anything when you're ganking a solo lane. Use that ult to get a lane or two fed. Camp mid if they're an immobile mage or get your ADC fed by camping the enemy ADC if their support doesn't have spammable hard CC. Warwick is also good at farming so you can just farm when your lanes are pushed up, then use that money to buy wards so you can see where they're going to gank and do a proper countergank (this applies to other junglers too). Vi is a good alternative to Warwick. She doesn't have as much sustain but her ult serves a similar purpose and she has way more hard CC.

Basically what you want to do is get your lanes as fed as possible. Trying to solo carry with Yi or some other champion isn't a reliable way to climb in the jungle because it depends on a lot of factors lining up in your favor. If you want to carry your way up the ladder, play mid or so.

Grab vi, look up XJ9 "dat vi" build. Carry, gg, profit.

You have to be completely selfish to play his style though. Which is perfect for low elo

Amumu is my main jungle champion. His ult is amazing. I'm not sure about the build you are working with but myself I can usually carry with him in a game.

Gromp, smite after to make sure the kill is secured, use potion, get blue buff, kill scuttle, get wolves, back, upgrade item and buy another potion you should have 2 potions for a full clear.

you dont need to be selfish but if you can secure a kill do it. i farm very aggressively on vi and i dont recommend building her pure ad warrior and black cleaver is all you need to do damage. then build tanky.

Pick ww or xin. Both are ez mode no skill required. Then just play a ton of games jungle till you figure out patterns of play and learn when you should and shouldn't gank.

Learn Rek'sai and Elise. Rek'sai brings a ton of map pressure while Elise is a solid jungle from early to late game with few weaknesses

Try Xin Zhao. He's super easy with great early pressure and sustain. You can start ganking as soon as level 2-3, as long as you have your Q and E. You can max your abilities in almost any order and be fine, I do QWE and R when I can. QEW probably works too. also solo dragon at 6

When lanes are pushed, you can either counter jg or work to prevent ganks. Counter-jg is more complicated and risky. Just ask yourself this question, if you run into their JG while countering, what can you do? You'll have to get a feel for that. Biggest tip here is try to figure out where the enemy jg was most recently seen and gauge your decision making on that. Preventing ganks on the other hand is easier and will help your teammates not die. This means kill crabs to get vision in river, ward the intersection between river and raptors on enemy side, ward enemy jg to know which lane the enemy jg could gank, etc. Think how you can prevent your lanes from dying by giving them more vision or knowledge of enemy jg positioning.

For ganking lanes that are getting pushed hard, I would prioritize the lanes that can carry harder if fed a little.

For warding, smite the wolf camp so you can "ward" your jg. I also like to put a pink ward defensively in early/mid game in the bush behind the red camp.

Just a couple of things I could think of off the top of my head. I personally don't counter jg well, so I stick to preventing ganks and preventing counter jg. Also, Yi is one of my favorites too! Try to pick Yi in champ select if you know they don't have hard cc (stun, lock down ults, etc.). So, for example, if they go a Vi or Leona, don't pick Yi. Past that point, just play yi a little passively before 6 and then start to get fed to stay relevant. Yi needs someone else to initiate and blow a few of the first spells and then he can go in and start. Try out Udyr, Vi, or Sejuani for jg also.

Good Luck!