I have heard that taking vitamin D3 will help my psoriasis. Is this true? If so, how much should I take?

Check a level. Get your vitamin d level checked. If you are low, then getting well into the normal range may help your psoriasis a bit. Vit d is also related to energy levels, depression (when low), and it may be an emerging risk factor for heart disease. So, lots of reasons to keep it normal.
Slightly. It requires very high doses to have any effect. Topical vitamin d such as dovonex or vertical helps psoriaisis but will not clear it. It can be used in combination with a topical steroid. Since vitamin d is a fat soluble vitamin it is stored and high doses can potentially result on side effects.