I would second using Monahans Radar Book for base learning. Would add that you'll also need to use your instruction manual for your particular gear.
Reason for suggesting a self learning approach is there are many tidbits to using radar that are only occasionally useful so if you try to learn them all at once before you have actually absorbed the base operational approach, you'll likely forget them (at least that's how I learn).
My knowledge is dated so modern radar has probably solved many of the issues with older CRT sets, but inexperienced operators invariably over-tuned sets to eliminate all clutter and thereby obscure all but the juiciest targets.
I also think it takes a while to get a feel for what zoom level to operate given conditions. This is coupled with shifting the screen. For example, my go-to setting for offshore transit on my 7-kt boat is 6-mile range, but I'll shift the center down 2-miles so I have about 8-miles in front, and 4-miles behind. Occasionally I'll zoom out to scan horizon or tinker to see how far off a target is (mostly out of boredom). Of course, in tighter quarters - a river perhaps - you'll want to zoom-in more.
Once these base skills are known, you can layer the cool features atop even though AIS has rendered many obsolete. MARPA/ARPA, bearing lines, ranges, etc. are still fun to tinker with as it gets you comfortable working through the commands in your particular set
In working through, you'll have to decide whether radar /chart overlay works for you. Personally, I'm not a fan - I'd rather run a split screen (one chart, the other radar) than a single overlay, but most people prefer overlay.
Bottom line, in my opinion, there are some basic skills that the Radar Book can assist with - tuning being number one, even if your auto tuning is satisfactory it's still good to know difference between gain and clutter/rain. It will give you the confidence to start learning and improving. From there, shift to the instruction book. For my personal style of learning, much of this is best suited to longer passages offshore where i can set aside navigation responsibilities for a few hours. Finally, be selective about the advice you receive and accept. Some areas of the country are more prone to conditions that require radar (pacific coast comes to mind). Others are not (Florida). Many boats have radar, but relatively few operators know how to use them suitably - and they don't often know what they don't know.
