First, explore what job possibilities there are in your community. I would recommend you network with dietitians who practice sports nutrition in your area. Find out what market there is for another sports nutritionist. Are there professional sports teams or a sports medicine clinic where you live or a college with a Physical Education or Exercise Physiology department where you could teach.
Second, would suggest you join SCAN which is the sports and cardiovascular nutrition DPG (dietetic practice group) of the American Dietetic Association. SCAN has a great newsletter.
Third, I would recommend taking sports nutrition CEUs (continuing education units) from reputable people like exercise physiologists or sports dietitians at your local university, community college or sports nutrition conferences.
Lastly, Melvin Williams has written a good book “Nutrition for Fitness and Sport”. Read it or “Exercise Physiology” by Frank Katch, Vic Katch, and William McArdle.
Good luck with your career change.