If you don't have a Clymer manual yet, get one. Also the Honda shop manual, which you can download as a pdf file, I believe.
O-ring is pretty much a modern standard by now. Some of us still use non O-ring, and that's fine too. Non O-ring should cost you a little less, but require a little more regular maintenance.
You will need either a chain breaker tool or a grinder to get a link off and open the old chain. Motion Pro is one of the reputable companies, of many, that sell them. I don't personally like the little ones suitable for roadside repairs. I like a heavier tool that will live in my garage. You'll be changing chains and sprockets regularly over the years with your bike; might as well have a decent set of tools.
BTW, what about your sprockets? 'Common wisdom' is that sprockets either get changed with every chain, or every other chain. Depends on who you talk to, and who's paying.
But if this is the first time, I would definitely consider sprockets along with the chain. Simple job. They bolt straight on as replacements. Old sprockets will wear a new chain unevenly, and likely cause an earlier need for replacement.
As for the chain, there are plenty of reputable brands out there. I've got D.I.D O-ring on mine, just now. You'll have to choose what kind of master link you want. I personally don't like clips; but they've been successfully used for a very long time, including on the racetrack. Somehow, I never seemed to get them quite right. A rivet master link is as secure as a regular link on the chain, but you'll need a special tool for that. Personally, I thought it worth the investment and peace of mind after losing two clip links due to my ineptitude. There is also a screw on link, but I've not heard them ever mentioned on this forum.
To get the old chain off, you'll need to loosen the chain with the adjusters at your back axle. Remove one link, and the chain comes off. NOW, some folks attach an open end of the new chain to the old one, and use the old one to thread the new chain onto the front sprocket. Me, I like to work with everything open. Besides, you may want to replace that sprocket now anyway. So, link off and chain off. Front sprocket cover off. Note exactly how the sprocket is mounted for your model. The 77 and 78 K bikes are straightforward. Now's a good time to take some rags and Simple Green and clean up that sprocket area. I'll bet it is full of 30 years worth of gunk.
I'm typing off the top of my head, and don't recall how many links you need in your chain. (Same as mine...) If it is a few links too long, you can easily remove a few. If it is too short, well...that's a little harder, though fixable. But more than one master link isn't really good.
Mount the chain around your two new sprockets. (Again, that's just my advice.) Pull it around to where you can work with it. Place the master link, press on the other half, use the rivet tool (assuming you got the rivet link) to secure it, and you're good to go for thousands of miles of riding pleasure. The 77 K is a great bike.
That's your primer. Use the search function to find the previous threads about chains. Some good info already covered, and worth your reading.
Also:
http://www.motorcycle.com/products/all-about-chains-3524.htmlGood luck! More knowledgeable folks will speak up here soon.
