Introduction
ID3 tags are the de facto standard for music tags, and for good reason. They’re incredibly versatile, with hundreds of different information containing fields. The standard supports tagging standard information, like title, artist, genre, date, and year. ID3 tags also support more advanced tagging, like BPM, label, mood, and much more.
There are hundreds of possible things which can be tagged under the ID3 standard (this document contains a full list). This means that in normal use, you could never use every possible tag.
Where Do I Start?
You start with the basics. I recommend starting with song title, artist, year of release, album, track number, and genre. These tags are universal and supported by just about any music player. These are also the things that you're most likely to sort by when you're looking for something to listen to.
Images
ID3 tags allow a variety of images to be embedded in the tags. The most obvious use for this is album art. Album art can make it easier to view your collection at a glance. Many music players and library managers will also display the album art of the song that's currently playing, so it can really improve how your collection looks.