Sometimes anger comes from a different root cause than I am describing here, but with many people forgiving others their trespasses as well as forgiving yourself for any mistakes is a good start.
Try thinking of happy things, good memories in order to calm you down. It took me a looooong time to get my anger in check, but it's possible. Deep breaths work too, and of course someone being there who can calm you down
take a walk
talk to someone
exercise
and if that does not work, 420