First, think about the topic of your site and write the words he would use his car in the search for a similar site. Now, if the sites that you expect will happen then you have a good list of keywords ready to build your site content around.
Do not make your content too heavy, but the key word, as this will result in a fine search engine, there are plenty of free tools available for you to check your keyword density and they will tell you what state is in place regarding the situation keyword.
Another good tip is to check your competitors who are getting good search results, view the source code on your page and take note of your keywords, now take a look at the content of the pages to see what kind of list of words are key. Do not copy a keyword competitors and the content itself, but use it to your advantage. A lot of websites tend to take the foot off the gas once they have a good search engine position. You have to put the time in this constantly to stay ahead of the game.
Use Google webmaster tools are there to help. Make sure your meta tags are correct, poor meta tags can affect more than you think. Once you have a good density of keywords, then you need to start building traffic.

Finding a keyword that describe your webpage is not an easy task. You can use google adword and wordtracker tool for finding the suitable keyword

Yes,  this is one of reasons outsource exist· To make it easy for those who can't afford the time and know-how.

Right keyword can increase backlink and traffic on the website and there are main two components for selecting right keyword relevance and popularity. Select keyword according to business and select popular or famous keyword and include at least 3 keyword per page in your website.