Think as a user and what keyword would you like to type to find your site. After deciding what keyword to target try to use keyword suggestion tool. Better if you would target a keywords that is not too competitive but has high search volume.
Start mapping out synonyms about your site and go from general to more specific. Then use a keyword tool. I like the SEOBook.com Keyword Tool the best. So start making a list like this furniture rustic furniture rustic mexican furniture Then I will take some of these new adjectives and maybe start a new list rustic chair rustic mexican chair Don't go to crazy just get those description words that really target your site/product. then use the keyword tool to find out what is actually being searched. so you could start with rustic furniture in the tool and like magic there is the long tail for that particular phrases. Be creative, get into the mind of your user, remember that there are a multitude of ways to say what your site is all about.
can you tell us whats your site so that we can easily help you get the right keyword for your site...
always going for non competitive and relevant keyword. software technology.software development...... otherwise you can check on www.digitalpoint.com. adwords keyword suggestion tool
You will know if the keywords is too competitive on the number of search results when you type your targeted keywords on the search engine.
This is all part of market research and identifying who your customers are. What are the consumer keywords for the market you are targetting?
How many search results is a good number to stay with and pick up a keyword? Below 1.000.000 search results or lower?