I'll give you some basic deets to get you started, though I've intentionally left out bits cuza I want you to read up!:
Runeterra: The main planet where this all takes place on. Has three known continents: Valoran, Ionia and the Southern/Shuriman Continent. The majority of League's cast originate from somewhere on Runeterra.
Ionia: An island continent with an abundance of magic. In more recent history they were invaded by the Noxian Empire, and have been reeling from that conflict ever since, with big shifts culturally as to how they're going to move forward as a nation.
Noxus: A massive empire originating from Valoran that has only grown with the centuries. They conquer other nations and incorporate them in to their empire, bolstering their armies with the unique strengths and tactics of the incorporated armies. Noxus is the largest single nation in Runeterra at the moment, and a lot of what they do has global implications.
**Demacia:**A robust nation formed in the aftermath of the Rune Wars. Using a material of their own making called petricite, as well as through their incredibly well organised military they have established themselves as a powerful kingdom but are fairly insular as they've been beset from all sides for a very long time. They're very wary of magic and those who use it.
Shurima: An expansive desert that encompasses most of the Shuriman Continent, it once was home to the Shuriman Empire; a vast nation that utilised the Sun Disc to transform the greatest among them in to immensely powerful beings known as the Ascended. Though this Ascension ritual ended up dooming their nation due to the actions of a man named Xerath, recent events have brought about the ressurection of Shurima's last emperor Azir, who Ascended and is ready to restore the empire once again.
Piltover and Zaun: Zaun was built on the only land bridge between Valoran and the Shuriman Continent, it tried to use a unique technology known as chemtech to create a cannal through the land bridge to allow for sea trade between east and west, though it went horribly wrong and sunk a good chunk of the city. Zaun endured and Piltover was built on top, and through regulating that new trade route Piltover has gotten incredibly wealthy over the years, which led to the creation of a new way to utilise magic for mankind: hextech.
Bilgewater: A coastal city on the Serpent Isles, Bilgewater is a haven for pirates, criminals and illegal trade. The city is in a dire situation presently with the deposing of Gangplank, as now the gangs of the city are unruled and going at each other's throats. It doesn't help that the Shadow Isles has it out for them either.
The Shadow Isles: An island chain that was once an idyllic natural paradise thanks to the blessed waters beneath the isle and home to a society of scholars, the Isle became a place of destruction and undeath after the Ruination. Now the Black Mist that covers the Isles manifests across Runeterra, claiming souls to continue growing itself.
The Freljord: A massive expanse in the north of Valoran, the Freljord is a daunting, unforgiving place. In ancient times the Frozen Watchers came here and transformed numerous humans in to their servants, imbuing them with power and naming them the Iceborn. The Iceborn served the Watchers until one among them, Avarosa, led a rebellion, which led to them being thrown off of a huge bridge. The Iceborn had their own nation but this crumbled in a civil war known as the War of the Three Sisters. Nowadays the Freljord's tribes are scattered, with the Avarosa tribes being led by a woman called Ashe who wants to unite the Freljord in the name of finding peace for themselves. The Winter's Claw is their primary enemy and led by Sejuani, who want desire unity as well but are willing to do what Freljordians do very well; killing and destroying, to achieve this.
Mount Targon: A remote mountain known for being a death sentence to try and climb, it is connected to a Space Faring Empire called Targon. The Aspects of Targon sometimes merge with humans who climb the peak, granting them great power and knowledge. Targon is a potent power beyond Runeterra, having gone so far as to enslave one of the mightiest beings in the known universe: Aurelion Sol.
The Void: A dangerous realm that few known much about, looking in to the Void drives most to madness. Beings from the Void are known as the voidborn; monstrous alien entities that consume as part of the endless cycle of creation within the Void.
Bandle City: A strange place that mortal men just can't seem to find, Bandle City is the home of the yordles; a species of small, furry humanoids with magic in their veins. Bandle City is really weird, as are the yordles, and while many still live in the city, some come beyond it to live among humans, using magical glamours to conceal their true forms.
