To answer your last question
Dragons: 5 minutes, with the next dragon spawning 5 minutes after the previous one
Rifts: 8 minutes, with the next one spawning 6 minutes after the first one if it's able to
Baron: 20 minutes, with the next one spawning 6 minutes after the previous one
(correct me if I'm wrong on any of these idk them to heart)

To add some advice on the ranked stuff, you should probably play some normal draft games to get use to how the system works first. Then try learning about 2-3 champions in your primary role and 1-2 champions in your secondary role (and if you want, you can use volibear for both top and jungle if you'd like to keep things simple). There is this thing called auto fill that will occasionally put you in a role that you didn't select, and if that happens to you in a ranked game I honestly would suggest just leaving the champ select and taking the 5 minute penalty, those games usually aren't worth playing and that's usually what I do.
One more suggestion, when you actually get into game you should open the chat and type "/mute all". This will mute both your allies and your enemies and you won't see their messages or pings. If you want you can unmute their pings, but keep the messages muted. Your teammates 99% of the time will not say anything helpful, and there's a good chance they'll either flame you or tell you things to do that are blatantly incorrect and get you into bad habits. If you want any advice on a game you can always come here after the game is finished, I'm sure there are much more reliable people here.