You need to do some keyword research, you can find a great eBook for keyword research on this website SEO Tips
Google adword is the best tool for find the best keywords for your website. https://adwords.google.com/select/KeywordToolExternal - 83k
Unfortunately there are no shortcuts - good research will pay off with good results. By entering a search phrase in Google you can see what sort of competition you are up against. The blue bar at the top of the results page might say something like 'Results 1 - 10 of about 4,160,000'. That tells you that the search phrase is fairly competitive. Also, you will learn a lot about how the search engines work by entering different phrases and seeing where they rank. Try and find combinations of keywords that return pages with poor information. If you can write a solid page of content using those keywords you should be able to rank highly in searches provided you follow the basics of search engine optimisation.
use google keyword selector tool and also check your competitor keywords and see how many traffic on that keywords and competitor for particular keywords.
I am definitely a big fan of Google's Keyword Tool as well. My suggestion would be to go to two or three of your top ranked competitors and check out their keywords. Then take like 10 of those keywords and enter them in Googles keyword tool (one per line). This should get you started.