I think it all depends on your comfort in the game. Also, as general advice, I would say not to worry about it too much, ranked is for you to play with people at roughly equivalent skill level, to help you improve on your own game. Generally, if you're feeling nervous, tired, or angry, I would avoid ranked (a little emotion before the game is fine, I still do this, but you want to be able to have a clear mind to focus on the game when you're actually playing). For the most part, you want to be at your peak when playing (ie. play a practice game vs bots or in normals, if you feel like you personally are playing well, your mechanics are good and decision making is sound, you can probably play ranked and do well).
Ranked is also a statistical sort of thing, meaning you shouldn't decide you're playing bad just because you lose a game or two. Maybe their team was just better, or some other factor outside your control. Over time, if you're playing well you should be able to win most of your games. Take a look at the win/loss ratio of challenger players and you'll see that most of them hover around 50-60% wins. And that's for the best players in the world, not to mention a lot of those players have smurfs, where the player will get really good win/loss ratios and only play on their main when they're doing spectacularly so as not to damage their ranking.
My advice is not to make such a big deal out of it. Everyone starts out somewhere in ranked. Nightblue3 (who's a very high elo jungler, he was #3 in challenger in season 3, so not a bad player by any means) has mentioned several times on his stream that when he first started playing ranked, he didn't even place in bronze (not sure if that mechanic is still in the game). If you place in plastic 8, no big deal, that's where you start, what's important is where you go from there. I got lucky in my placement games in season 3 and was placed in silver, but honestly was having a tough time getting even a 50% w/l record. This season I was placed in bronze, I've honestly learned a lot and improved a great deal, and actually just this morning got back into silver. Honestly, ranking doesn't really mean anything, it's nice as a marker of your achievements, but achievement doesn't mean anything if you don't earn it. The more you learn about the game, the better you get at it, that's what's really important here, your ranking will naturally follow your improvement as a player. And everyone gets flukey games every once in a while, it's your job not to let those get to you, or if they do, take a break. If you're good at the game, know how the game is actually played and won, you will win more games and you will naturally rank up; there's no golden bullet to guarantee 100% win rate.
Now, to actually answer your questions, I would say pick the champions you are most comfortable on, especially mechanically. Being very comfortable mechanically is incredibly important to develop to progress in ranked (not necessary to play ranked, but important to improve on as you play more ranked); the less you're thinking about your mechanics, the more you can think about the actual game. Knowing your champion inside and out, knowing their matchups and how you play with and against certain champions, as well as knowing how to build situationally depending on your team and their team is very important to learn (it's a lot, you don't have to know it all at once, or even when you start playing more ranked, but they're important goals to be working towards). Same thing goes with roles; play the role you're most comfortable as. Every role can carry the game if you know how (yes, even support). That said, it is a team game, and Riot seems actively pushing this aspect more and more (honestly you can carry a game easier with good shotcalling and decision making than you can with pure mechanical skill).
Finally, if you're serious about improving your game, I would start seeking out more information around the internet. You can only learn so much grinding in solo q. Best of luck!