I would like to become a life coach - I have a passion for helping
people lose weight through a controlled carb lifestyle.  I have had
some real success with friends and family and have been encouraged by
them to help others.  Do I need any type of certification for this
type of work?  If so, where do I go, what is the cost and why do I
need certification - can't I just call myself a coach if I am
helping/coaching others to develop a healthier lifestyle.

You can start your career as a life coach without having any
certification. However, some US states are beginning to pursue
legislation to regulate the profession. In that case, you might need
to clear an exam to practise as a coach.
Here is a website which will provide additional information. Good luck

http://www.life-coaching-resource.com/