I think taking a traditional course at ESRI or at a community college where they offer ArcGIS classes. I find this way the best way to learn and you can learn from instructor than the online course. If you are familiar with the computer side such as simple basic stuff like creating a new folder, copy and paste etc..
ESRI has their online course that you can take but you have to have the software installed on your machine and if you have a trouble how to fix or need some help it is on your own and you have to figure it out yourself and things like that...
That is just my opinion.