Collect weapons through daily quests and tournaments. They provide extra damage, and progress you to a team’s full weapon set providing even more bonuses! Once you get a weapon you will always have it!
Since weapon upgrades get extremely important later on (once you have every weapon you get Hero DPS x 10) we highly recommend you to participate in the tournaments. Tournaments are the best way to get alot of weapon upgrades (15 for Rank 1, check out the Tournament section for more information).