If you want easy the 450 champions are usually the safest picks in their respective roles.
If you want to learn to last hit then Shen is who I learned to do it on. Last hitting under tower is helpful and he excels at that job. It is a basic skill that some people just overlook.
Sona is pretty simple as far as supports go. Only 1 skill shot and can be played aggressively to poke and defensively to sustain with her healing. Utility comes in from her speed ability. Morgana works well too if you want to be more aggressive in this role. Land Q, shield carry, cast pool to zone out enemy's escape or to zone them from jumping closer to your Carry. R if multiple targets. Don't play Blitz support, he's so one dimensional that you really only learn how to aim skill shots.
Warwick is a safe pick for jungling but I learned to jungle on Udyr. Literally no skill shots, granted he has no "ultimate" ability. He's good at farming out the jungle and is simple to gank with (hint, you press E). He can go AP/AD style damage output based upon what ability you max first. Tiger is better at solo targets so things like killing the carry, doing Dragon and Baron. Usually this is what I go with. Phoenix is the AP version and he excels at clearing out lots of weaker enemies. He can split push faster, clear out the wraith and wolf camps quicker and most people don't expect this to be your maxing skill. Bear stance is used for mobility and CC while turtle is good for sustain and for defense on that shield of his.
Mid lane Annie is quite easy. Her kit is very straightforward. But you can use her bear to face check bushes for you instead of doing it yourself. Can be used as in place of warding but warding is still highly recommended. That being said, Morgana is a nice mid laner to learn on. She is the one I used. Granted, her "farming" is super easy (Aim and press W) so if you're trying to learn last hitting she's not a good choice at it. If you do like Morgana Lux is a very similiar champion. Lux is all about skill shots though so if you need practice at aiming she's your girl. It also helps that she has super low cool downs. So you can get more opportunities per game to work on your aim. If you do play Lux You can press E a second time to explode it early. And her laser has a pretty short cooldown too, so spam that. Use it to snipe runners, use it to annihilate a minion wave, use it to snipe blue buff and dragon. It's pretty versatile. Karma is my personal favorite mid laner. Her ultimate ability is available at level 1 and it empowers one of her other abilities. This allows you to react one of three ways to certain situations. AOE poke/zoneing on her Q, W is her sustain/tanking and E is her group shielding/mobility. I find her fun and interesting.
ADC is not my best role. I recommend avoiding both Draven and Twitch. Who I would recommend using is Caitlyn. Her stats are pretty mediocre but her range allows more safety in lane. Just don't spam Q and you will be fine on mana. Now onto more specifics with this role: Want to learn last hitting? Quinn is pretty good with that passive. Want to learn combos/positioning? Ezreal. Want to know what it feels like to win a 1v5? Vayne.
Top lane I mentioned Shen but you can also go with either Garen or Darius. Learn on one and you're halfway to learning the other. Darius is more offense based while Garen is more defensive. Renekton is also a strong lane bully and is real fun to play as. Jax is good too. He will teach you auto attack cancellations by timing your use of W you can attack 3 times in the time it takes for your opponent to attack you 1 or 2 times. Jax is also one of only 3 champions who can ward hop (Lee Sin and Katarina are the others). Throw a ward over a wall and Q onto it. Will be sloppy at first but with practice you can ward and hop in one fluid motion. Avoid Gnar, seen too many bad ones to say he's worth learning.
I didn't name any assassins. As far as roles go that role is the hardest one to learn. But if you insist on learning that role start with Master Yi. Press R and just auto attack the enemy to death. Press Q to dodge (seen too many bad Yi's press Q to initiate) and stay on someone. Nidalee is all about aiming that spear and using cougar form to farm up. Fizz is similiar to Yi. He dodges with E, repositions with Q and adds on the damages with W. He is also one of the only assassins who has an initiation tool. He is a good pick for low ELO solo queue since you are the only person you can rely on in that environment a majority of the time. Katarina you press R to become a mass murderer. Syngergizes best with Morgana. Use Q to poke, W for added damage/small mobility boost and E to reposition. Also can teach you the basics of ward hopping.