The ring bearer is a young boy (but now a days it can be a girl) between the ages of 4 to 8 that walks down the aisle before the flower girl (or boy). They carry a pillow with the bride and grooms wedding rings on it. If you are not the trusting sort, you can tie fake rings to the pillow and give your maid or matron of honor or the best man the real wedding rings.
When choosing a ring bearer, look for a young family member or a close friend of the family to serve. Make sure that this child is comfortable with people looking at them. To ease them into this, make sure that they are at the rehearsal dinner so that they can give walking down the aisle by themselves a try.
As an added tip, you might want to have a special designated chair for the ring bearers parents, close family member or sibling that will be able to assist if the ring bearer starts to become entertainment during your ceremony. The ring bearer in my wedding, is one of my close friends son's who is also one of my bridesmaids, so if he starts to need assistance he can go and stand with his mom.
If you do not have young relative or a friend of the family..you can also use your family pet. There is a current trend in having a pet be the ring bearer. If using a pet, please make sure that you have trained them on what to do and have someone at the wedding to care for the pet during the ceremony.
When choosing a ring bearer just be sure that you will be upset with the child or the pet does not do exactly what you want them to do...it would not be a wedding if they did everything right.