There are plenty of free tools that can help give you ideas on keywords. They can tell you the most searched terms or how difficult a keyword is to optimise for etc. But you should also start with an analysis of your own products and services in order to fully understand your capabilities and potential.
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Keyword Analysis
The following diagram provides a snapshot of a web user’s search methodology and demonstrates how less frequently searched terms can actually yield a better return on investment than the more competitive core terms. This is largely due to their proximity to the purchasing decision.
Web user search process
At the earliest stages of web search, very broad [...]