You can also donate to the global fund to raise money for the families impacted by the shooting. It's already almost at its goal, though, so if they don't need money anymore by the time you get there, your local mosques are always doing great things and helping people, so your donation will be well-used.
As always, stand up against Islamophobia. Demand an end to hateful rhetoric that inspires white supremacist terrorists to murder. Shout back against those who promote and spread that hateful rhetoric (yes, I'm talking about PewDiePie because yes, this is partly his fault).
Don't be tricked into silence by the idea that this hateful, murderous monster did what he did to promote a "culture war" and therefore we should stop criticizing people like PewDiePie. There is already a war. So much blood has already been spilled. Allowing the spread of poisonous rhetoric is what got us here.
Don't try to interpret the impossible web of irony and meta shit in that monster's "manifesto." It's not worth it. Continue with what you know to be true - white supremacy and Islamophobia kills and therefore must be ended.
My Muslim neighbors, siblings, and friends have a right to be here and have a right to exist anywhere they want. They are an essential and wonderful part of my community and I will fight anyone who tries to hurt them.