Our best source of vitamin D is sun exposure. The body synthesizes "D" using ultraviolet (UV) rays absorbed by the skin, and very little sun exposure is needed to provide your body's needs. Even if you live in an area such as Pennsylvania where you get little strong sun in winter, adequate exposure during the rest of the year will allow your body to stockpile enough "D" to last you through the gray winter months.
So, someone who has fair skin, burns easily and lives in Pennsylvania would be advised to spend 20-30 minutes in the sun with your arms and legs exposed (not your face) between the hours of 11 a.m. and 3 p.m. two to three times a week from March through May and September through October but only 15-20 minutes in July and August when the sun in strongest. If you remain outdoors longer than the specified time, apply sunscreen. (Don't even try to get sun time from November through February; in your area, you won't get enough UV to help, and you'll probably get pretty cold outside with bare arms and legs.
The intensity of UVB rays is also reduced by clouds, pollution and UVB will not travel through glass, so sitting next to a window will not give you enough sunlight to make vitamin D.
Vitamin D isn't found in many foods. You won't get any from plant-based foods unless it's added to them. Vitamin D is naturally found in oily fish like tuna and salmon, but most of the time its added. Milk, soymilk and breakfast cereals are usually fortified with vitamin D (and maybe other nutrients - check the label). You can also take vitamin D supplements, which are often found in multiple vitamins, calcium-vitamin D products, or as single nutrients. Some vitamin D supplements are labeled as vitamin D2 or vitamin D3. Although your body absorbs more vitamin D3, it doesn't really make much difference because you'll get enough with either type of vitamin D as long as you follow the directions on the label.
From The Vitamin D Solution by Michael F. Holick, M.D., Ph.D. ----you can find more info at http://drholick.com/