You can find most of what you need on YouTube. Or join online fb communities Sugar Geeks, Cakeheads.
A few other very detailed, for a fee, sites are: Yener’s Way, Bluprint/Craftsy, and Cakeflix. If you are interested in a specific technique, in your search engine, put in the topic. Example: If you want to learn how to cover a cake in fondant, enter “Covering a cake in Fondant Tutorial” and you will get lots of free hits.
I second both June’s and Sandra’s suggestions :) All and any would be excellent choices. The question is what do you mean by “professional”. All of these sites will teach and encourage you to master the craft, but to the best of my knowledge none give any kind of certification to show that you are “a professional”or that you have studied a specific number of hours or that you have achieved excellence in any of the cake decorating / design fields. Again, if it is knowledge that you are seeking, any and all of these suggestions are excellent.