There are numerous ways to create an online avatar, and among the most popular in 2015 are two sites: Avachara.com and SecondLife.com. Both are free services, but the styles are vastly different and typically used for different purposes. The best site for you depends on your intent of use.
Avachara allows you to create an anime style avatar for use in chat rooms and forums. You can also download an image of the avatar that can be uploaded to any compatible host. You begin by choosing gender, and from there, you can customize physical attributes of the face and body, as well as clothing and style.
Second Life is a highly popular virtual world where you can create a digital version of yourself. Compared to many free online avatar creators, the quality and realism of Second Life is currently second to none. Not only can you create an avatar in your image, but you can shop an extensive marketplace for accessories and even join the online community in an interactive virtual world.
IMVU.com is another free avatar creation platform, but it requires that you create an account with the company in order to access the full library of customizable avatar pieces.