Best sex position for first time sex?

No such thing. Most positions have no physical risk. All sex involves intimacy and personal preferences. I think the official medical answer is there is no best position for first or other times.
Protected. This is a magical event and should not be rushed. You should be in full control, not under the influence and in a relaxed and comfortable place where you will be comfortable. Missionary position is generally the best for a gentle start. And take proper precautions.