I have a lot dead skin build up on the bottom of my feet right under my big toe it hurts so bad when standing are walking for so long on it.

Dead skin. There are several reasons that you would have this and you can try an exfoliate from the drug store. I would not recommend that you cut this off yourself. I would also try a pad to relieve pressure to the area and see your local podiatrist. He/she will fix this for you.
It . It sounds like you may be suffering from an irritated callus. A callus is merely a large area of thickened dead skin. In some cases it can become irritated, mainly from rubbing against your shoes. You can try over the counter callus removers. If this faisl you may want to see your primary care provider or even a podiatrist(foot doctor) for a complete evaluation and treatment of your problem.