If you feel light-headed when going from supine to standing, the most likely cause is dehydration. Make sure you replace fluids and salt lost in sweat. Other causes can include anemia or heart-related issues, the most common of which is a benign condition called mitral valve prolapse.
If your dizziness feels more like the room is spinning, or your head is spinning, and is accompanied by nausea, you most likely have vertigo. Vertigo is fairly common and most causes are self-limiting and benign; the exception being tumors of acoustic nerve, like acoustic neuroma (itself a slow-growing, benign tumor.)
If you have vertigo, find a doctor who is familiar with the Epley manuevers. (Google it for an explanation.) Many cases of vertigo are caused by little crystals building up in the labyrinth canals of the ear, and the Epley manuevers dislodge the particles and clear the cause of the vertigo.
This is probably TMI but the big thing is first ask yourself if you have dizziness or vertigo; they are very different conditions.