Hm... Riven is a really heavy snowball champ. If you're not snowballing, you're not winning. I'm a meh Riven and tbh she has more tough matchups than people lead on to believe. Anyone with a decent understand of Riven mechanics can outright beat any DECENT Riven player on just about any champ.
You can join a few co-op vs AI matches and try out a few champs. You should also stick to 3 champs like a lot of people say (I don't follow this rule because I'm alpha af -- kappa (also why I'm stuck in gold 1).
Champs I think are really good tanks and still deal tons of damage:
Trundle: This can be your go to pick for ANY melee matchups. Trundle just outright takes out all melee and tanks, and is quite tanky himself. I'd say this would be a good go to champ to learn
Gnar: I would love to suggest Gnar to you as a really safe top laner that deals damage and has a lot of team utility. I loved the champ before he even became meta, but right now, considering he's meta expect him to be picked often and get ready for some nerfs (I'll suggest another one, this one is an iffy suggestion, you might actually want to stay away from it)
Maokai: A really safe pick, great farm potential, awesome teamfight potential, great damage, easy to use kit. This guy is just great, you get a handle on him and your lane will be a junglers paradise. You could legit just push top straight to the inhib with your junglers help and they won't be able to do anything to stop you (from my experience in silver -- been there a year -- people really don't know how to handle tanks). I'd say Mao is a really strong pick
And a really good Niche pick to learn and my all time favorite for those pesky meta champs:
Pantheon: This guy just demolishes just about every meta top laner, just, whatever you do, don't pick him into juggernauts or ranged champs. He has an amazing early game and if you can learn to abuse his early game, you pretty much just force ff's all around. Really good person to learn for those pesky riven/yasuo players
And Malphite will be my last suggestion for the top lane. I know, I know, most of these guys are tanks and you want to do damage. However, all of them are powerful and have a lot of impact. Trundle is great for splitpushing, Malphite/Mao is great if you feel like you actually have a time that can teamfight well. They all fit great into their own niche comps and learning all of them will give you enough champs to body top lane however you see fit.
As for jungle, I can't help you out there. I still struggle there myself. I'd say, stick with junglers that scale well into late game, bring team utility and do NOT require snowballing (Lee, Kha, Elise, Rengar) < I get tilted every game when I see one of those junglers. I'm normally scared they won't perform well, but I do suggest you learn ONE of them so if you're last pick and the other team decides to go a full team of squishies? Boy, I sure love me some Rengar in those comps. Free kills, free stacks, free win.