What matters the most is the relevancy of the keywords you choose rather than the density. This is actually basic marketing: you have to understand what and how your potential customers are searching to be easy to be found. The easiest way to do it is to ask yourself that question: «What product/service/informations do I sell/offer?». The answers has to be really accurate to be relevant.
Then you put yourself in your prospective customer's shoes: what is he the most likely to type in? We recommend you to chose 2-3 sentences for your keywords instead of one-word ones, it will send you much higher quality traffic. Also, be careful to chose neither too specific nor too vague terms. And eventually, if you hesitate between two words variant or word order and can't afford both, pick the one that seems the most popular among users.
You can look at those used by your direct competitors in their Titles and Meta Descriptions to find some inspiration. Using tools, such as Google Adwords or Google Trend, is also a good thing for it provides you keywords and reflects what the user seeks and so the Search Volume. The Search Volume is indeed essential. As a matter of fact the more people look for a word the more you get to reach a large audience.
On the other hand, if too many websites are competing for the same keywords, you might rank lower than expected. This is why you should be aware of the Long Tail keywords. These are 4+ keywords phrases with low competition. They are responsible for 70% of the traffic over the Internet and bring few but engaged users to your website. It's up to you now to find the balance between too competitive and not relevant enough
And finally, being well ranked is not permanent: it has to be updated. You must keep expanding your keyword's list! Next time we'll tell you everything about how to position your keywords to be as well ranked as possible.