You only need about 10-15 minutes a day of exposure to the sun! if you live in the north then in the winter it's tough! You need to get a blood test to see if your D3 is up.
Also I should have added, take with a meal that contains fat, as that helps fat soluble nutrients get absorbed, and make sure it's natural Vitamin D3, not some synthetic version carried by some drugstores.
Just going out in the sun at peak hours should do it unless you're having some problem synthesizing it. It's fat soluble, so it stores in the liver, meaning it should register fairly quickly and stay up unlike water soluble vitamins that leave the body quickly and have to be replenished more rapidly. One of the main reasons for low Vitamin D these days is we've got lifesyles that avoid sunlight and we've also been told sunlight is bad for us, but that's how the body gets Vitamin D naturally.