Congratulations! Being a father is one of the great joys and responsibilities of life.
Have she been to see a doctor? If not, that is the first thing.
If she does not have one, buy her a book on pregnancy, or look at the many web sites. Search for 'pregnancy week-by-week'.
You need to clean up your life style. Eat healthy, cut out all drugs and medicines, start to prepare your home for a baby. Be attentive to what she needs and wants. Help her around the house with chores and make sure she rests and relax enough. She also need to stay fit, so join her for walks if possible.
Ask her what you can do to help her!
Tell her every day you love her and are going to look after her and the baby. Tell her how wonderful she looks and how much you are looking forward to the baby and how good a mother you think she will be. It is important that she feels loved, secure and totally unstressed and relaxed.
Join her for her doctor's visits and ask questions if you do not understand or want to know.
I am sure other people will add more things you can do.
Best of luck!