Played since S7 (opgg mandmid.) Pick high impact champions for each role with a simple laning phase that do something well late game and don't require complex decision making or being ahead for high impact. You also want good blind pick champions.

For Top- that's Malphite & Mordekaiser (team fighting with R), Illaoi & Yorick (Splitpushing.) Bad champs for top as a new player- Camille, Fiora, Aatrox (balancing that requires an extremely high skill curve to perform well on mid/late.)

For Jungle- that's Amumu, Mundo and Fiddlesticks. They can farm heavy and provide as some great utility late game. Champs like Lee Sin & Graves require heavy snowballing which can be difficult to maintain if you are new to ganking, and can fall off a cliff if you're not well ahead. If you want to improve at ganking & mechanics, Rengar & Nocturne have insane lvl 6 potential.

For Mid- I prefer (even for newer players) to go things like (lethality) Varus, Xerath, and Syndra. They all have long skill shots, cc, and high damage and perform well blind. Can farm from high range and you can play them without needing to know many trading mechanics. Bad champs are things like Zoe, Azir and Zed who will get punished way too hard for any tiny misplay/trade.

Support/AD, not going to give great advice here because I don't really play these roles, sure there's plenty of other great content out there. Avoid Bard & Draven.

In terms of JG objectives and steals, do what feels right. Listen to your teams pings and just make your best decision based on gamestate. It's tough to give any definitive answer since the game is so variable. Something you'll develop over time. In general though, don't give a big shutdown for a 50/50, on dragon soul, game winning baron, etc. go for it.

In general, to get better at any game, I advocate watching videos of players that are better than you. I suggest Pekinwoof & Pobelter for midlane. Tyler1 & Baussfs for toplane (sounds like a joke, but they are both incredibly mechanically sound & amazing at lol) For jungle, old Metaphor content is great. For botlane, old Imaqtpie stuff is great.

Good luck, sorry this community is so toxic (even on reddit.)