Well that is a good question. Our main source of Vitamin D is from the sun but if you live above the equator then you are likely not getting enough. People who fake and bake tan get sufficient levels of Vitamin D in most cases. Supplementation is generally the only other way to obtain it. It takes several months to build the levels back up.
I read the book Vitamin D Cure and depending on what your deficiency level is at you should be taking anywhere between 1000-3000 IU of Vitamin D a day. Your body weight is also a contributing factor to appropriate dosages so the larger you are then the more Vitamin D you need and vise versa. It is one of the few nutrients that stores in the body so you can take it all at once for the whole week or in seperate dosages each day. Also, try to spend approximately 15 minutes or so in the sun several times a week.
If you have dark skin you are likely to have difficulty taking in Vitamin D levels from the sun as well as if you find it easy to tan. It is actually people with very white skin that get Vitamin D more quickly from the sun.
For more accurate dosages I would read the book "Vitamin D Cure" I highly recommend it.
I hope this helps
Do you have any medical problems ?