Again, though there is no "right" website for social bookmarking, I prefer Diigo as I believe it provides the best functionality for teachers.
To start, create an account on Diigo. Go to https://www.diigo.com/ and follow the instructions. Click on Get Diigo Now. Free! in the lower right.
You can always upgrade later if you decide there is value in doing so, but for now choose the free plan.
Enter a username, email address, and password. Click Create Account.
You will have to verify that your email account is yours by going to the email and clicking on the link.
Before logging in, an important first step is to make it easier to add items to your library. In the upper right toolbar there is a Tools choice. Click on that to get the tools to make your job easier.
This will bring you to a list of tools.
There are two tools in particular you should look at now. Both the Diigolet and the Chrome extension make it much easier to add content.
The two tools are similar. Clicking on my Chrome extension icon for Diigo, I get a menu:
Clicking on Save brings up a screen to input a description, keywords - here referred to as tags, and the opportunity to add it to one of my groups.
The Diigolet, on my toolbar, is slightly different, but still allows me to easily create a bookmark.
Once you have one of these tools installed, adding resources is quick and easy.
Now let's login. Once you are logged in, you will see that you have a few bookmarks already. These are there to help you get started. Notice that the list of tags is on the left, and will grow as you add tags to your bookmarks.
Before going any further, however, note that some of the functionality we want to use - such as groups - is reserved for premium plans. Fortunately, like many quality websites, Diigo provides teachers with free premium accounts. So you will want to go to the Premium page and apply to get an educator account. In my experience, it is straightforward to get such an account from these types of websites. Click on the Go Premium button in the upper right, then look for the text at the bottom.
Click on Apply on the far right under Teacher.
Note that the educator account allows you to create students accounts, making for a better teacher-student experience.
In the meantime, let's look at your account now to see how to add bookmarks. Click on My Library in the upper left to come back to your library page. The main page of your account shows your bookmarks.
You can add bookmarks using the tools, or come to your library and click on Add.
Choose bookmark, then paste or type in the URL (web address).
Click on Next to bring up the entire record where you can add tags and a description, and add it to a group.
You don't have any groups yet, so let's create one. At the top of your screen, click on My Groups.
In the upper right, there is a button that says Create a new group. Click on that.
You give the group a name and a description, then add some other settings information. Note that once you have an educator account, you will be able to create 20 private groups with 50 members each, more than enough for your classes.
Now you are on your way to meaningful social bookmarking. Once you receive confirmation of your Educator's account, you should set up groups for each of your classes, as well as look for public groups that you want to join.