If you're not sure why "Black Lives Matter" is the appropriate and just thing to say, not "All Lives Matter," then please read this, and here's an excerpt from the Black Lives Matter mission statement:
We work vigorously for freedom and justice for Black people and, by extension, all people.
We intentionally build and nurture a beloved community that is bonded together through a beautiful struggle that is restorative, not depleting.
We are unapologetically Black in our positioning. In affirming that Black Lives Matter, we need not qualify our position. To love and desire freedom and justice for ourselves is a prerequisite for wanting the same for others.