Considering you're quite new to the game I'll just glance over the basics with you. There is a Meta but there are also tons of exceptions in the game so I'll be pretty general with how I explain things. The game, and the MOBA genre as a whole, is quite difficult to get into so the best thing I can say is just play the game. You'll learn the most by doing that. Feel free to ask any questions and I'll get back to them.
Positions:
Positions are generally separated into the "lanes". Generally you'll have two people on the bottom lane (The AD Carry and Support). Mid is in the middle lane and Top is in the top lane. The Jungle position doesn't have a lane and roams around the map clearing jungle camps while supporting lanes as well as trying to control the map, buffs and Dragon/Baron.
Ranged AD Carry (Attack Damage Carry): The ranged "Physical" damage dealer, or "Marksman". One of the bottom lane positions. Meant to farm as much gold as possible and win duels with the opposing bottom laners if possible. Really strong, consistent damage late game with their auto attacks. Ranged attackers such as Vayne, Graves, Ashe, Caitlyn, Ezreal, Corki, etc. are considered AD Carries.
Support: The other Bottom laner alongside the AD Carry. Supports the AD Carry in getting them farmed and keeping them alive. Will give all the last hits to the AD Carry to ensure they get as much gold as possible. Also tries to set up kills if possible for the AD carry. Buys the most wards for the team to prevent ganks early on and also for more map control later on. Champions such as Janna, Soraka, Thresh, Leona, Sona, Blitzcrank, etc. are considered supports.
Mid AP Carry (Ability Power Carry): Considered the "Mage" or "Assassin", the mid laner is a solo lane that has a similar role to the AD Carry. Generally a 1v1 duel in the mid lane, you get lots of farm but your skill set is different. Your damage is more "Burst-ey" than the AD Carry with the majority of your damage coming from your skills as opposed to the AD carry which uses mostly their auto attacks for damage. AP Carry's will also come equipped with more spells that have "Crowd Control" (CC for short). These are things such as stuns, slows, etc. Not always Ability Power based though since there are assassins such as Zed that can be in the midlane but opt for AD/Physical damage instead. Champions such as Kassadin, Ziggs, Zed, Lux, Syndra, Fizz, Ahri, etc. are mid oriented.
Top (Bruiser/tanks): Top lane is another 1v1 duel lane. Usually will have a tanky melee champion in the lane but can also opt for some mages as well (Vladimir is an example). Top lane champions probably have the most variety in terms of what can be used. For instance, Shen is a full tank who goes top lane and is great due to being abel to taunt enemies and shield teammates later on but lacks damage while a champion like Riven is more duel oriented to winning the lane and can provide more damage to a team later on but lacks the team oriented skills/defense Shen does. Top laners generally have more survivability than the Mid or Bottom carry due to being naturally beefy. Champions such as Singed, Shen, Gangplank, Nasus, Jax, Irelia, Rumble, Riven etc. are Top oriented.
Jungle: Jungle's the hardest to explain. Also my least favourite role haha. In the jungle there are camps of neutral minions that the jungler will farm in order to level up and get gold since they don't have lane minions to farm. Much like the top lane, junglers come in all different forms but they will generally be bruisers similar to Top lane. Main difference is one of the Summoner Spells they take called "Smite" which does true damage to minions, speeding up farm time and also giving the ability to steal important minions such as Blue/Red buff, Dragon, and Baron. They support the whole team and can provide map/ward control, buff control, and also gank lanes. Ganking a lane that is behind can help get your teammates back in the game or can help keep certain champions shut down by ganking the same lane repeatedly. When playing in the lane, the jungler is what you want to be wary of if you start to overextend towards your opponent's tower. It makes it easier for the enemy jungler to come in behind you to gank. Many junglers will have skills that help in ganking such as a dash, stun, slow, etc. Champions such as Nocturne, Lee Sin, Xin Zhao, Nunu, Shyvana, Amumu, Jarvan, Hecarim, Udyr, etc. are Jungle oriented.
Not sure what else you need help on but feel free to post and ask what you need clarification on
