I have been experiencing pain in my left foot mostly. i usually get it when i just get up in the morning or if i sit down. what could be causing it?

See your doctor: Problems like these can only be correctly handled by your doctor in person. He/she needs to listen to you, perform an examination and possibly run labs or other tests. That's the only way he/she can find out what's going on and what to do about it.

Many things: Pain can arise from foot strain, injury, arthritis, circulatory, nerve issues among other things. Getting a good history and examination, and perhaps x-rays can help provide a more specific answer and help you begin treatment. See a podiatrist.