What keywords should I use?

This is one of the most important things in search engine optimization. The crawler, when he visits your site, has to know what your site is about based on these keywords. First of all you have to choose a primary keyword phrase . This phrase should be the phrase most searched. Don't try to choose a phrase too competitive because it will take a lot of time to rank well and you will probably have to buy a lot of links. I you have a business that sells used cars don't choose your main keyword "cars". This generic terms were taken a long time ago by smart people(and rich too know). You have to choose a program that tells you how many searches a keyword receives. I use http://www.keyworddiscovery.com/keywords.html . It even has a free trial. Using this program I have found out that "cars" receives 1,329,755 searches, "used cars" receives 414,996 searches, "second hand cars" receives 3,421 searches. Even for 'used cars' is almost impossible to rank well if you don't plan on buying links. Your target visitors should be the people from the city where you sell the cars . That's why you need to rank better for that state/country and one way to this is to include the name of the city/state in your keywords. "used cars new york" gets 456 searches. Once you have chosen your primary keyword phrase you should think about other secondary keyword phrase (derivative from your primary keyword). Always look if the plural form of the keywords are more searched than the singular form. Google knows its still the same word but you will rank better for the the form you choose to optimise.