How do I tag my question?

I have come up with a question to ask on CodeChef Discuss, but I am confused on what tags to use. Can you give me some tips on tagging a question?

A tag is a keyword or label that categorizes your question with other, similar questions. A tag should be relevant to the question being asked.

If you are asking question related to a particular contest use the contest code as the tag. If you are asking a question related to a problem use the problem code as the tag.

Some good tips on selecting a tag can be found here.