You would have to stay outside naked for like five hours to get enough vitamin D synthesis this time of year in some states when you are really lacking, or sit or lay naked in front of a window with sun shining through for at least 10 minutes a day.
There simply may not be enough sun where you are in February to get all the vitamin D you need, so you need to go shopping. A glass of milk a day will get you 10-percent of what you need. Fatty fish like salmon is also a good source of vitamin D. But your best bet might be a supplement, a vitamin D supplement will replenish what your body uses and loses in the winter.
I don't think supplements should replace vitamin D rich foods. The pills should be considered an addition to your diet.
Winter does that, also it's a time to keep warm
Or go out and play