Mentally prepare yourself to get destroyed in your games early on. The gap between the difficulty in normals (where people are generally on relatively unfamiliar champs and/or somewhat screwing around with weird builds) and ranked (where people are mostly on their best champs and usually trying their best) can be pretty jarring at first, but try to stick with it. It's easy to go "woah I'm not ready for this", but really the thing that makes you good at ranked is playing ranked.
It's possible that you've had enough normal games that you can hold your own at a silver-ish level out of the gate, but it's extremely common for first time ranked people to end up in bronze at first. The key thing is to not despair about this and not to blame your teammates (who'll inevitable seem awful) and have confidence that you will eventually rise up if you keep playing your best and learning from your mistakes, and that sort of attitude will carry you far.
For more concrete advice, make sure you've got a full rune page of appropriate runes for each of the champs you intend to play in ranked, and try to have at least 2 champs per role (ideally more) that you're confident in so you don't get stuck with your main banned/picked and no alternatives.
Since you barely adc/jg, I'd avoid putting fill as one of your roles. Since you main support, you'll probably get that 90% of the time when you queue for ranked, particularly if you put it as primary, and you can put mid/top as secondary.
