There are a few ways to rephrase this, but they depend on the context of the word 'something' in this sentence. 1) You can't believe in anything. 2) You won't believe what happened. 3) You shouldn't believe everything he/she says. 4) You can't believe everything you see/hear. There are more options to rephrase this, but again it depends on the context. I hope these at least give you a better idea. ~Samantha :)