Follow the guidelines below on when to use upper or lower case. Remember: overcapitalisation can make your text cluttered and harder to read, especially on screen. When in doubt, use lower case.
The basic rule is that proper nouns have an initial capital but common nouns do not. Initial capitals are often employed to excess in commercial and administrative circles, but they can be visually distracting and are often unnecessary, so should be used sparingly. When in doubt use lower case.