Where can I learn to use Final Cut Pro X?

For an excellent introduction to FCP start with Steve Martin’s 60 minute intro. 

Learn FCP in Under 60 Minutes - YouTube

There is also Final Cut Bro getting started 

 https://youtu.be/L3bZ-ETKZ8U

For a more in depth introduction use the Izzy video tutorials. The version is a bit dated but the principles are the same.

Final Cut Pro Tutorial | Izzy Video

If you want to do structured courses, then I don't think you can beat Ripple Training. Tom mentioned the Steve Martin YouTube tutorial which is a good sampler for the Ripple Training courses. I'd suggest checking that out and then checking out their courses if you like that. They are not free but are very good value in my opinion. They often run discounts on their courses as well. I think there is one at the moment with 35% off.

where should I start an online course?

There is "Help" on the left side of the menu. A user guide will be displayed that explains all the features of FCP. Get into the habit of looking up the user guide when you don't know.

The tutorial shown by Tom is helpful. However, it is impossible to remember all the contents. Please verbalize and record what you have learned so that you will not forget it. It is important to record things in writing rather than relying solely on memorization.

It's been over 3 years since I started learning FCP. I continue to record the important information I learn and create my own personal guidebook. Information is organized by item. I'm searching for information when I have a problem or can't remember. This guidebook is saved in iCloud so you can refer to it anytime, anywhere.