When creating a team from the ‘Troops’ menu, select the ‘Hero’ button from the top-left of the screen, this will bring up the weapons you’ve earned so far. Select one and place it in your team. Bam! Hero in team done!
Troops are an important part if not the most important part of Gems of War. You can level them, unlock them, and strategize around them. It’s important to have even mana distribution among your team to make the best out of their spells, powerful attacks they can unleash by matching corresponding gems and powering your troops.
Simplifying it is no easy matter, and there’s a lot to learn. But I will leave you with this wonderful guide which should come in handy (given you give the time to read it):
If you’re looking for something a bit simpler or more comprehensive, try this one (though all provided info may not apply for the console variants of the game):
If you need help with another or more specific question, feel free to leave a reply!