In general, hyphens are joiners. Use them to avoid confusion or to form a single idea from two or more words: much-needed clothing (clothing is badly needed) versus much needed clothing (the clothing is abundant and needed). Do not use them in pairs to create an em dash.
Hyphenate compound modifiers preceding a noun: well-run establishment, ill-fitting garment, full-time job, smoke-free restaurant. A compound modifier following the noun it describes does not require a hyphen, but it is not incorrect to use one: The restaurant is smoke free. When a modifier that would be hyphenated before a noun occurs instead after a form of the verb to be, the hyphen is usually retained to avoid confusion: The man is well-known, The woman is quick-witted.