Generally speaking you want to start out by picking champions that you enjoy playing, because they're who you'll naturally have an easier time with even if they aren't strictly the best power picks you could go with. Start with that while you get more comfortable with build paths and playstyles, and eventually open yourself up to trying champions who have kits you aren't quite as familiar with or enjoy quite as much, but can make a much larger impact on a match.
You'd be surprised at what you learn to enjoy once you've mastered the basics with someone else. For instance, when I started learning top I did so with Shyvana back in season 3 when she was in a better place than she is now. I thoroughly enjoyed her kit, and I still do, but eventually I had to admit that while she was really fun for me to play, and I had learned how to use her to disrupt enemy teams and protect my carries or mop up fights, she had no real utility built into her kit to capitalize on outside of a meh knockup from her ult. So I branched out and started learning champions with a bit more capabilities, like Riven (RIP season 3 Riven), Wukong, and Teemo. Since then I've switched from being a top main to a jungle main, but I've made sure that I'm at least competent with most top laners.
Outside of what you enjoy, or can learn to enjoy, you do need to pay attention to who's popular and why they are. Renekton can be a powerful pick because he can out-sustain most other top laners while still dealing a good amount of damage. However, he falls off in the late game pretty hard. On the other hand, someone who's more worried about a late game comp might go with Jax, who starts off a bit weak but rapidly hits his power curve around the midgame, and can go hyper carry by the end of one. The main reason you want to know who's strong and why is because you want to make sure you can counter where possible, and that means you need to have more than just two or three champions available. It's a good starting point, but you'll naturally accrue more you feel confident with as time goes by so don't rush yourself.
It's good that you're already considering versatility, because that's one of the main things the top lane is there for alongside the jungle. Midlane will always be a squishy AP caster, be they assassin or mage. Bottom lane will always have a glass cannon ADC and a support with varying levels of tankiness and utility. But the top lane, and the jungle, both give opportunity to play pretty much any role in the game provided it syncs up well with your team, and you don't counter yourself. This is where a deeper champion pool really comes into play. You want to have multiple champions you can play for any role you might have to in the top lane, because you never know what your team might end up needing during champ select, and you won't know who you're fighting until that point either. An enemy that one fighter might be weak against another might be strong against, and you have to be ready for that. As well you need to make sure that whoever you pick has synergy with your allies if at all possible.
I should probably point out to you now that there's no such thing as having one role help the team and carry more than another. I can't tell you how many matches I've carried as a tanky Volibear in the jungle, as a slippery top lane Teemo, or as a wall of "you aren't touching my team" with support Thresh. Who and where you're playing isn't as important as how you're playing. There's a good reason Riot removed the name "carry" from champion descriptions: anyone can carry with any champion provided they know what they're doing and work with their team. I'm not saying you're wrong for wanting to play mid, but don't hold yourself back with that sort of a mindset.
Don't main Riven mid though lol. The idea got a bit popular during season 4 but fell off just as quick. It CAN work, but only in certain situations.
But yeah. The main things you need to worry about are whether or not you enjoy a champion, their current standing, and whether or not they can work with their team. Let your champion pool deepen at it's own rate and you'll do fine.
Why not play the Pirate?! Here's a guide 
