Just log in starting April 25
Blizzard is looking to bring new players into Heroes of the Storm with next week's major "2.0" launch. Things like Genji and Hanamura from Overwatch, a refresh of the progression system, loot chests, and emoji are all well and good. But even better than that? Free stuff. Loads of free stuff, in fact.
By simply logging into the game between April 25 and May 22, you'll earn 100 Gems which you can then spend on a Mega Bundle. There will be four of these themed bundles, each with 20 (!) characters.
The idea is to choose the bundle that best suits your playstyle (or perhaps just has the least amount of overlap with your current lineup). There's one for assassins, one for supports and specialists, one for tanks and bruisers, and if you're unsure, there's the "bit of everything" Flex bundle. Keep in mind, you only get to claim one. After picking a bundle, "the other three will become unavailable."
For those of you who are thinking of coming in fresh, this is a hell of a head-start.
Mega Hero Bundles [Heroes of the Storm]