TL;DR It's mostly trial and error, but over time you will figure out how you want to organize things. Keep in mind that once you do this, for it to remain useful you will have to keep up with it. Also, don't forget that you can add tags to smart folders (saved searches) you've created. Edit: to answer your question, I have over 500 tags, so I don't think you would want me to list them here! You could start general, then get more specific. Like, start with a Business tag, place everything business related there, then later add tags to more specific things, like finance, marketing, sales, advertising, etc.. You can reference wikipedia.org subject glossaries for ideas on how to organize things in this way.
I recently did this, organizing everything and categorizing everything with tags. It's nothing to sneeze at - it took me roughly a year or two because I took breaks, and worked in sprints when I felt like it. It's really worth it once your done, but if I could go back, what I may have done differently was just started tagging everything new that I brought onto my mac, rather than slogging through everything from the past. Granted, overhauling the organization of the entire system taught me how to go about tagging things - it takes a while to get used to. Either way, it really pays off in the end (so long as you keep up with it), and it's something I think everyone should do.
It's hard to say what tags you should have, but my tags have changed a lot from when I started to where I am with it now. It's a lot of trial and error as you start to realize that you have been trying to organize things based on preconceived notions you had of how things should be organized, and how you actually want them to be organized. The best approach I can suggest when doing this is to figure out which files you have the greatest quantities of, and determine how you want to have them sorted. Then, aggregate them all in a folder, or start a custom search, and search for each term individually that describes the greatest number of your files - if you've filtered your results to the types of files you want to tag, you can just select all and tag them all. Do this for all the terms you found that make up a large swath of files. Like, I have a lot of PDFs i want to be able to locate, but I don't want to tag the PDF, I want to tag the folder it's in. So, I create a custom search; add a new criteria, name, i enter the name; add a new criteria, kind, enter folder (or select folder?); this way I only see folders with that name.
What I have found tags tags to be great for is keeping things organized that have overlapping relationships. Like, do I keep my router documentation with my networking and communications documents? Or with maintenance documentation, firmware and scripts? With tags I can just tag things as 'router' then create a smart folder that searches for the tag 'router', and have the 'router' tag, or the 'Saved Search' folder on my sidebar. Then when selected, any file related to routers pulls up; notes, firmwares, documentation, etc.. It's so nice to not have to figure out where something is, or to forget that you have some useful information you downloadded but never organized. Especially when you're working... like, I will tag documents that are useful to a project, with that project name. So, every time I pull up that tag, my whole workflow is right there, and I didn't have to worry much about which folders things are kept in.
While you're going through this, in the mean time you can get pretty effective results in the search bar by entering "name:" following by your search terms - without the quotations - and it will pull up any item matching those search terms. You can also open a new search by hitting option+command+spacebar, then click on the gear and open search criteria to place as many filters as you want.
Hopefully this helps you on your journey. Good luck!
