1). Start playing normal draft pick.
Choose two roles to be good at, since you will not always get adc. Playing draft pick as adc primary should give you adc most of the time. You only need 2-3 champions for each role. More is good, but at first, get good with 2-3. Also practice one support to be at least OK with for your ELO (doesn't have to be good) for times you get autofilled. (if you are support main, just support is OK due to how autofill protection works, but have 2-4 support champions).
Once you start carrying your weight in normals, you can move on to ranked.
2). Ignore toxic people. Once someone calls you names, just mute them and ignore them. Take advice from non-toxic people (anyone who does not insult you explicitly) seriously. Sometimes that advice is good, sometimes it is bad, but that's how you learn. Do not turn on /all chat at first (it is toxic at low ELO most of the time).
3). If you main a CSing role (adc, mid, top), practice CSing from time to time in custom. Start a custom games, use normal runes/masteries. Your goal is 29 (80%) of CS in first 6 waves (5 minutes in mid lane, a bit longer in others), for bronze-low-silverish. If you are way below this number, practice CSing more (perhaps once CS per PvP game you play, on average), until you can consistently hit 29+ (in higher ELOs that number in higher, for Plat I use 34, but learn to walk before you run).
You can add me if you want, I can give more advice. It won't get you to challenger (I am not there myself), but can help you get started.
Most importantly, ignore toxic people/trolls, they are not worthy of attention. You'll lose some games due to them, but you'll win more due to them being on the other team, since your team is guaranteed one non-troll (you)!
