Just FYI, when you start Ranked the very first time, the system consider you're roughly Silver3, and then moves you up or down depending on your results. Whereas on Season reset, it still partially takes into account your old rating, it's a "soft" reset. So if you think your actual skill level is very far from Silver3, either way, it might be a good idea to start playing Ranked during pre-Season, just to get it to a more accurate level before the real season start, because even with the best match-making, it still takes a bit of time for your MMR to be properly evaluated.
Alternatively, you can try to start right now and snag the Silver Loading Screen Border for next season :p Because when you virtually start in Silver3, it's hard to "demote" all the way down to Bronze in just 10 Placement Games, so most people who finish their placements for the first time end up in Silver5 or something, even when they actually have a Bronze level. As I said, estimating someone's MMR takes a bit of time... (I'm half-joking, but if you like the "collector" side, at least you'll be able to say "I was here in Season 7" ;) )
For Supports, I happen to be a Sona main, and I have to warn you, she's mostly a heal bot too. Some people build her with Tear/Lichbane/Ludens and try to burst squishies, but that's honestly a terrible idea. A good Sona will poke a lot in lane, but past laning phase, it's a mistake to move up that close to the enemy team, so you can't abuse your Lichbane, and you end up with a dead item in your inventory. In the end, it's much better to build as a pure utility Support. I do love the champ, and she's pretty accessible on the mechanics side of thing, but if you hope to make big plays, Nami is better. Sona wins through lane pressure, extensive vision, and proper map rotations, plus proper positioning and staying safe in team-fights and spamming that heal + 5-man-shield, it's deep strategically but it's not flashy.