1. Use cautiously. US English uses hyphens less frequently than British English, and Canadian and Australian English usage is somewhere in-between.
2. Most compound words do not require a hyphen. When in doubt, look it up. A copy of an up-to-date dictionary will be very useful. Merriam-Webster’s Collegiate Dictionary, 11th edition (suggested by Goss and Goss) for US writers. Macquarie for Australian writers, Oxford for UK writers.
3. Use hyphens to separate letter sequences that distract the reader from construing the word correctly. g. co-workers.
4. A word or phrase used as an adjective is often hyphenated, but it may not be hyphenated when used as a noun.