There's a coop mode against bots in LOL in which you can train a bit before heading into PvP. Actually, when you make a new account you are forced to play at least one match against bots before being able to play competitive matches.

The first thing you have to do when jumping into an online match is to pick a role. You do that by typing the role you want to pick. There's no in-game mechanic to pick a role, that's why you have to type which one you want and talk to your team about it. There's an unwritten rule that says that the one who types first is the one who will get the chosen role. For example, if i am the first to type "top" i will be the one going to the top lane and no one else can pick it up. Of course, since it's pretty much an unwritten rule created by players there's no way to enforce it and there might be some trolls every now and then who will pick their role even though someone else had already chosen it firs, but most of the time people follow it.

There are 3 lanes in the game: bot (bottom), mid (middle) and top. There are 5 roles in the game: top, mid, adc, support (both go to the bot lane) and jungle (called jg). Top is the one who will go to the top lane and is usually a melee champion (but not always). Mid is the one responsible for the mid lane. Jungle is the one who plays in the jungle between lanes. ADC (attack damage carrier) is the one who deals damage and should be a ranged champion (like Vayne, Ashe, Sylvir, Caitlyn and Draven, for example). ADCs play in the bot lane. Support is the one who will support the ADC, as the name says. These are the champions that can provide shields, heal, pull enemies, use crowd control spells. Sometimes people go dual top instead of having one in the top and one in the jungle, especiallyin matches with lower level players, but it's really not recommended.

Each champion is better suited for one particular role (and lane). You need to pick a role and then pick a champion suited for the rolw you have chosen. However, some champions are more flexible than others. For example, Yasuo can go mid or top, it's even viable as a jungler. Blitzcrank, for example, is not recommended for any role other than support.

I don't recommend support and jungle for newcomers to the game, since they do require quite a bit of experience, especially jungle. In the jungle role you need to know exactly which neutral monsters to slay, when to gank, how to counter jungle, which are the best positions to put wards. Avoid the jungle role until you get more experience.

One of the most important things in LOL is to make a build that suits your champion and your current situation. You do that by buying items in your base (there's a merchant near the spawn point). LOL already has a tab that shows you the recommended items for your champion, and it's accurate, for the most part. But these items are also situational. For example, if the enemy team is dealing a lot of magical damage, you whould buy an item that provides resistance to magical damage (depending on your role). If you're suffering from crowd control spells, you should buy items that mitigate the duration of such spells.

There are many guides online that show the best builds and the best way to play with certain champions. Watching matches on YT from experienced players is also a great way to learn and improve your skills.

Well, that's just the basic. You'll only fully understand it after playing it for some time. I still have a LOT to learn and i've been playing for quite a while.

If i played in the NA server i would play with you.