Dear you have not given your age ,sex or any other medical conditions for which you are taking any treatment. You have mentioned that you feel dizziness when you stand from sitting position . This is suggestive of autonomic dysfunction that is when you stand there are few changes that pushes blood against gravity to your heart that can be pumped to brain and other parts of body.in some illnesses this system get failed or is not so active as is required . Diabetes is very important cause apart form this dehydration certain antihypertensive drugs etc can cause this. We need complete history and examination to confirm the diagnosis . In the mean time I would like to suggest you that you should take care of your self while standing from supine or sitting position . This should be slow and with support . Drink plenty of fluid and stop any hypertensive medicine thar can cause this after discussion with your physician . Hope this will help.