Move all pictures, videos and music you don't need available all the time. If the phone takes a microSD card, put them all on that. Copy anything else to a computer, laptop, cloud account, somewhere off the phone. (
Anything important should already be backed up to someplace off the phone.)
*
Go to
XDA Forums, find the forum for your phone and see if anyone has a list of bloatware you can safely remove. What's safe depends on the model phone and the carrier. Most manufacturer and carrier software can be removed, but not all.
Don't depend on that being a permanent fix, though. If you keep putting more things on the phone without taking things off, you'll run out of space again.