Do yourself a big favor and spend some time navigating and reading the website of the Vitamin D Council, a non-profit that's not peddling anything but free education out of concern for a population that's increasingly dying of things people never used to die of.
The high rate of natural production of vitamin D3 cholecalciferol in the skin is the single most important fact every person should know about vitamin D—a fact that has profound implications for the natural human condition.
Technically not a "vitamin," vitamin D is in a class by itself. Its metabolic product, calcitriol, is actually a secosteroid hormone that targets over 1000 genes in the human body. Current research has implicated vitamin D deficiency as a major factor in the pathology of at least 17 varieties of cancer as well as heart disease, stroke, hypertension, autoimmune diseases, diabetes, depression, chronic pain, osteoarthritis, osteoporosis, muscle weakness, muscle wasting, birth defects, periodontal disease, and more.
Vitamin D's influence on key biological functions vital to one's health and well-being mandates that vitamin D no longer be ignored by the health care industry nor by individuals striving to achieve and maintain a greater state of health.
I'm pressed for time, otherwise I'd reference my pasts posts on D. Just enter 'vitamin d' in the Google search field in the right sidebar and that should find them all for you.
And BTW, the most convenient form of supplementation I've found is the Carlson, 2,000 IU gel-caps. They're teardrop size and I take three per day, just like is sitting beside the bottle.