Every time i stand up my legs shake uncontrollably until i start to walk then it stops, why?

This sounds like: A Parkinson's like disease or syndrome. You need to see a neurologist ASAP. This can also be due to basal ganglia ischemia (read remote stroke) or other less common movement disorders. Sometimes this can be a medication side effect as well. how long has this been happening and what other symptoms do you have. Have a family member take you to your doc ASAP. New onset today? ER now.