My Samsung Galaxy S5 space is full? And I have NO idea why?

I only gave five apps and can't downloaded anymore nor can I delete those because they're the ones I actually need every day. I only have like 20 pictures and can't even GET to those because the phone has locked me out of them until I clear some out (even though i have done this and the sd card I bought keeps deleting them). I have ZERO movies, music, videos, etc., because my phobe deleted them all including the ones of my 4 month old niece and I'm getting extremely frustrated. I don't have a computer to back these up on and I don't have the space to download an app at all, and I cannot change phones. Please someone tell me what I'm doing so wrong and how to fix is ASAP.

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