You can read up on it in the book by Dr Holick. He recommends 4000IU daily, but if you're afraid that's too much, go with 2000IU/day. I take 4000IU daily since several years. Get a test before you start, so you have a base value. Test again after a month or two maybe three, and see what the change is.

With sun exposure, the book recommends the following. Go sit in the sun on a sunny day, until you get a mild rose burn. How long does that take? Divide that time over a week, and you're safe. This rose-burn time depends on the time of year, how high is the sun in the skye? How clear is the skye? What is the time of the day? So this value changes a lot for people in norther Europe, compared to people living in Africa.