I would say to start out with the tags for your questions itself - it should be pretty easy to figure out.

As answers come along, they may introduce you to new terminology, etc. that is related to the question. As tags are primarily for sorting, it would be best for the question to be sorted according to others like it.

For example, if I ask a question about Foo, and someone answers with something like,

Oh, you mean Baz (a more common name for Foos). Bazes have the... (Answer)

Then you might consider tagging your question with baz, as it is another term.

However, if you ask the same questions and the reply is something like:

The reason that Foos do that is because they have a bar that is 5.628.

Then you should not retag your question with bar.

It's a little complicated, but it's also fairly common sense.

I would say to start out with the tags for your question itself - it should be pretty easy to figure out.

As answers come along, they may introduce you to new terminology, etc. that is related to the question. As tags are primarily for sorting, it would be best for the question to be sorted according to others like it.

For example, if I ask a question about a planet outside of the solar system, and someone answers with something like,

Planets outside the solar system are commonly referred to as exoplanets...

Then you might consider tagging your question with exoplanet, as it is another term useful for sorting the question.

However, if you ask the same question and the reply is something like:

The reason that certain planets are like that is because they have irregular rotation.

Then you should not retag your question with rotation, because it does not help sort the question. The question does not really involve rotation beyond the fact that it is key to the answer. In other words, someone searching questions would not know to search for rotation unless they already knew the answer, so it's not a useful category for that question.

It's a little complicated, but it's also fairly common sense.