Becoming an editor and/or proofreader means you must have an excellent understanding of grammar, spelling, punctuation, style, formatting and layout.
Obviously, no one can remember everything connected with these things so it is important to have a good set of tools.
Tools required are:
1. A good localised dictionary (ie it’s no use owning a lovely thick American dictionary if you reside in the UK or Australia).
2. A marking pen (ie standards change from company to company, so be sure you know what colour is used for each position, such as editor, copyeditor, proofreading or publisher).
3. A localised style manual.
The above are the absolute minimum you must have.
Australians might considered subscribing to the Macquarie Dictionery Online. However, there is an annual fee to access the information.