Are humans basically just brains controlling a body ? Or are we really one living being ? Also, when we make a decision, is it always just the brains that decide or do we also have our "own" will ?

clearly we are living organisms with many organs that play vital roles - one of which is the brain and nervous system. In most cases the failure of an organ leads to serious illness or death.

The second part of your question really has little meaning - as individuals our brains make decisions which are influenced by multiple factors and inputs from the rest of the body. The concept of "free will" as being separate from the brain/body has no validity.