Here are some of the major gaming platforms that has millions of gamer's base. If you are not counted in the list, download one and use it.

Steam

Let's address the elephant in the room. Steam has been in the forefront of game publishing for over a decade now. Steam has over 67 million active monthly users. The amount of PC gaming market controlled by steam is over 60%. With over 20 thousand game titles varying from AAA to indies, steam has it all.
you can download the steam client for PC, Mac and Linux using the official link down below for free
https://store.steampowered.com/

Origin

Origin is EA's very own game client. All the EA titles are accessible through origin. Headliner titles like Battlefield, FIFA, Madden etc are available through origin. Origin may not be as huge as steam but it has the titles worth purchasing. Origin also offers a subscription-based service called origin access. 
download origin using the link down below for pc and Mac for free
https://www.origin.com/ind/en-us/store

Uplay

Uplay is Ubisoft's game client. All the major Ubisoft titles are marketed here. Beyond this Uplay offers you access to titles like Assassins Creed. However, Steam market these games too.
download Uplay client from the link down below for pc
https://uplay.ubi.com/

Battle Net

Battle.net is the game client for purchasing Blizzard titles. Games like Overwatch, World Of Warcraft, Diablo etc. The upcoming Call Of Duty title is going to be in battle.net
download battle.net from the link down below
https://us.battle.net/account/download/