Oke so there are 2 ways to go about this game. 1 to get better and have the potential of climbing and 2 just to have pure fun. I think it's best for you to decide which one you would favour more. Don't get me wrong you can have fun while getting better but sometimes getting better means you have to do some really (subjective) boring stuff.
Lol is pretty different from Hots. I haven't played that much on HOTS so I am not sure what most of the heroes do (sonya was my fav).
My advice is to just let go of everything that you've learned from HOTS and just try to build a solid foundation. The fundamentals of league are very different from HOTS (not 100% but still pretty different).
First you need to pick a role. Top/ mid/ jungle/adc or supp. You don't need to stick to 1 role but it gets easier to play the game if you stick to 1 or 2 roles. The easiest way of chosing a role would be looking at the champs that you enjoy playing the most and where they can go. A lot of champs can be played in multiple roles with the exception of most adc champs.
2nd play champs that are easy to play. Playing hard champions while new to League can be very annoying for anyone that is in your team. Most players even those who have been playing this game for years struggle with a lot of the harder champions. Playing easier champions like:
Annie Garen Soraka Leona Warwick NuNU Vi Lucian Caitlyn Amumu
These champions are pretty easy (some a bit harder than others) but are very straightforward. Champions that are straightforward are not only gonna help you build a foundation, but also enables your team to understand what your purpose is. Harder to play champs usually have their own agenda and require some work around.
Champs like:
Lee sin Zed Leblanc Akali Vayne Ezreal Riven Yasuo Corki Draven
These champs are either 2 hard to play or require a bit 2 much of their team to be fully effective. This doesn't mean that you can never win with them, but it is much harder to win with them consistently.
And last but not least, PLAY A LOT. Play the game as much as you can. THe more games you play the faster you'll pick it up and have a feel for it. Some games open up pretty fast and you might be put into a position to make a quick decision on what to buy, when to peel or initiate etc which can win or lose games.
Whenever I wanna learn/prac a new champ or role I use the following sites. As an example, I'll use caitlyn
First I'll go to her wiki page to see what she is and what her abilities do.
leagueoflegends.wikia.com/wiki/Caitlyn
Try to read things carefully as it can help you see patterns once you start playing other similar champs work.
At level 30 you should be able to fill up a rune page along with a mastery page. These pages will help you dictate a play style and can help you in difficult matchups. It's not the end of the world to not have the correct rune or mastery page but it could make the game harder.
For runes and masteries I search up this site and use it as a basis http://www.probuilds.net/champions/details/Caitlyn
You can do the same for items, but for now it's smart to just stick with the recommend items from the in game shop. ONce you feel comfortable you can move over to more complex build that are situational.
If I want more details in runes and masteries along with the items I go to this site
http://www.op.gg/ranking/champions/name=caitlyn
I search up the highest ranking caitlyn players in the korean server. The Korean server is widely considered the best in the world. I go and check builds and masteries and see what they do and sometimes watch their replays.
You don't have to do any of this atm, but it's only when you wanna become better are champion. Right now, you need to just play a lot and see what you feel like playing the most. I wish you the best of luck