Suggest The Dosage Of Vitamin D

I am a mom, have a 29 yr old that is on lexapro for depression and Vananyse(Sp) for ADD. BLOOD WORK REVEALED she is very low on VIt D. What amount of supplements; vit D should I purchase for her to take Would the other medications she is taking interfere with how the body would process the VIt D? How much VIt B would she need? Thank you , any help appreciated.