In order to unlock more fighters, simply playing the game will do the trick in most cases. You get rewards for logging in multiple times per month, as well as rewards for completing missions and Events. There are many ways to get it, but here are some of the more common:

Various stores: you can purchase pieces to unlock or upgrade fighters based on your performance in different areas in the game (Arena, Club, Challenges). My suggestion is to always hoard tokens and only spend them for getting Fighter shards. This could be considered the main way to unlock more Fighters for free

Complete Elite battles: there's a limited amount of these per day and you should always start with them because you will win extra Fighter shards. Raid for faster results and focus on the Hero you want to unlock/upgrade

Different Events: Keep an eye on those as well since there will always be events rewarding you with Fighter shards in the game.