Yes, you need to obtain the proper personal training certification to work with individuals one-on-one. Although it may appear easy to just transfer the knowledge you have as a group fitness instructor, there is so much more that goes into training individuals. For example: Assessment and program design (based on individual goals), anatomy, kinesiology, behavior change, communication, health screening/joint issues, and special populations and considerations all just skim the surface. Not to mention liability.
So, if you are serious about helping others with individual training, look into an NCCA accredited personal training certification. There are many to choose from, and good luck to you.
Best to you,