Assuming you don't know anything at all in the game....
The first thing you gotta do is pick your champion.
There are 6 main categories which each champion fits in and partially fits in: Assassins, Mages, Fighters, Supports, Tanks and Marksmen (but everyone calls marksmen "adc" which stands for attack damage carry).
An assassin is an agile champion that specializes in killing or disabling the enemy carry (someone who "carries" is someone of high value to the team).
Assassins are focused on infiltration, deception, and mobility.
Mages are mostly ranged champions who prioritize powerful abilities over basic attacks. Typically mages are characterized by some combination of long-range, area-effect or high-utility spells to get the job done.
Fighters are close range combatants that possess a mix of offensive and defensive capabilities. While they don't have as much utility as a tank or as much damage as an assassin, a fighter's damage will add up over time to make it a major threat.
Supports make plays by enabling their allies through buffs and heals, or by disrupting enemy lines through crowd control. From laning to late game teamfights, supports create advantages and opportunities for their teammates to capitalize on.
Tanks are durable, front-line champions that help lock down enemies and start fights. They're usually found leading the charge, choosing the right times and situations to initiate aggression. Many tanks can also protect their more fragile teammates by stunning or pushing around dangerous foes and limiting their damage potential.
An ADC (Marksman) is a ranged attacker that sacrifices defensive power and utility to focus on dealing strong, continuous damage to individual targets. Typically focused on using their basic attacks more than their abilities, marksmen have the capability to scale and deal out devastating levels of damage in the late phase of any game.
Which one appeals to you the most? Once you've decided, research some champions that fit in that category and find one that you like. Since you're new a cheap one is best for now. Play that champion and only that champion until you get bored of them or want to learn how to play different champions. In my opinion you should be able to learn on your own just by playing again and again or by looking up guides. Don't hesitate to ask for help from your teammates.
If you have any questions or want me to explain more then just reply.
[slayer-jinx-catface]