The first thing to do is relax. This is about the enjoyment of you and your sub. It is best to start simple, find out what you and sub would like to try. Set down the guidelines of what you expect from her and what she expects from you.
Some basic questions are:
Is it going to 24/7 thing or just casual?
What safe words will use?
What are your limits?
Plan out your first scene in simple terms, ie today we are going to try spanking etc.
As time will go by it should become more natural to you.
If you need any particular advice feel free to email me.