How do I get disc brakes on my motorcycle?

Any engineering problem can be solved with enough time and money, but never heard of a motorcycle conversion kit. If you want a modern bike, get one. BTW, there is nothing wrong with drum brakes.

The disc calipers need suitably strong mounting points on the forks and frame. If they're not already present as an option on that bike then structural modifications will be required to the frame and a replacement set of forks engineered to take caliper mountings. That's in addition to disc-equipped wheels.

On a forty year old bike it's highly unlikely to be worthwhile unless there is a complete insurance-approved kit already available.

It already should have a front disk.

The rear drum should be fine.

You will have to find a new rear wheel with exact same offset, then build a caliper mount/machine the swingarm so the caliper slides with the axle....

Then will need to fabricate a new brake lever setup/mount, then mount a master cylinder, route lines, then test it out.

Since about 80% of your stopping power comes from the front, the rear drum is more than enough for that bike.

You don't on that bike.
It wasn't designed for them, and nothing suitable is available.
I know I suggested you park it for a while, but I certainly didn't mean permanently.
It's old tech, not bad tech.
When you take the course, you will ride a modern bike.
I was hinting that you should get comfortable with riding before dealing with the (minor) differences. Drum brakes are not bad, I prefer them on the rear. Discs grab quicker, allowing you to stop easier.

BTW, my first was a '74 CL360....wish I still had it.

Don't bother.It will cost you more than the bike is worth.

check the fork tree part numbers, check out a set of later dual disc forks at a salvage yard, might find a set of lowers with dual disc from a 1980s Honda that will direct fit for a couple hundred dollars. I'm running mostly Yamaha front end and brake on 1981 CM400 Honda- works good. Harleys in early 1970s got 750 Honda front end put on them, bearing kit for the adaption of Honda tree neck to Harley frames. The sale of 20 bearing kits in a month roughly matched the 20 stolen Honda 750s reported and later recovered minus front ends in couple areas.

I don't think I've ever seen a Honda cb550 without a front disc brake. Yes, the rear had drum brakes.

The rear drum really isn't all that bad if it's set up correctly and maintained. The front brake, while pretty good for it's day, is downright scary today. It shouldn't be too hard to find a dual disc conversion or a single disc and caliper upgrade for the front end of your bike to at least bring it up to basic standards of today for braking.

Join a Honda CB forum online and look through the archives of the forum, or just ask a question there. Someone has done what you want to do and can help you do it too.

It should already have a single front disc brake, they also had optional mountings on the other fork leg for another caliper and disc mountings on the hub.

With that up front you really do not need to touch the rear brake.

You can refresh them with a better caliper, master cylinder, hoses and discs. Your suspension could probably do with a rebuild.

Get Disc Brakes