What do you intend to do with it?
If you just want to scan for hot spots you can figure it out. You can determine 'this thing is hotter than that thing'
On the other hand if you want to produce decent reports with temperature accuracy you will need to take a course.
I have been to a 40 hour and a 16 hour course with Flir. One thing you quickly learn is that there is much much more to it than simply turning it one and taking a picture even though it seems like a digital cam or video recorder.
The big issues are reflectivity and emissivity if you do not understand how those affect the result you will not get accurate temp readings.
I would like to eventually be able to render professional reports. I would like to eventually incorporate this into my business.
That being the case I don't think there is any question you need training and obtaining your level 1 certification.
I am not certified yet, most likely I will be in the near future.
I do not know anything about the online courses but I can tell you that the live classes I went to were interesting, fun, yet very technical and I am not sure I would be able to learn what I did on line. There were labs in class with tough IR issues to solve and you had a teacher walking around to help you through them.