I understand how you feel with your apprehension, I used to be the same way. You just have to go for it dude. Playing vs Bots is honestly a waste of time. Like others have said, custom games are only useful to practice clear speeds and jungle routes. If you're trying to learn how to jungle, then learning jungle should be your goal rather than winning. If you're worried about teammates having a bad reaction just mute them preemptively. You will only learn jungle through practice.
Also, when you're playing an off-role i.e not your main, it's usually better to just play what's easy (Vi, Master Yi, Amumu, Xin, Diana etc) or what's strong (Elise, Evelynn, Kindred etc). If you can get comfortable on some of these champs you stand a good chance of winning games. It may be tempting to play something like Sejuani/Nautilus but they're not great right now. Strong high CC junglers atm (especially for solo queue) are Amumu and Zac.
When you're jungling you need to have a different mindset. It's important to look at the lane matchups in champion select to analyze where you need to be applying pressure throughout laning. It's also important that you balance farming + ganking. By the time you're doing your second round of buffs you should be level 5 at least - this indicates you're on track.
A simple blue side route: Krugs (get leash from botlane) --> Red buff --> wolves/river crab --> blue buff/river crab --> gank top lane if you're healthy --> Recall --> Razorbeaks (smite for vision buff) --> Gank mid lane
Red side: Gromp (bot lane leash) --> blue buff --> wolves/river crab --> red buff/river crab --> gank top lane if healthy --> recall --> razorbeaks (smite for vision buff) --> gank mid lane
If you need more in-depth information there are plenty of youtubers who will be useful to you, like stonewall008 or foxdroplol. It might also be worth watching Nightblue3's stream, he's fairly informative and engaging to watch. It all just comes down to practice and familiarizing yourself with it, try not to get too wound up if you make mistakes - that's how you learn. Good luck :)
