you can use following tools: FREE: https://adwords.google.com/select/KeywordToolExternal http://inventory.overture.com/d/searchinventory/suggestion/ PAID: www.keyworddiscovery.com www.wordtracker.com This is an indicative list only.
first, use the above tool to do the amount of search done. Then use google.com to search how many results are there currenlty.
Keyword research is the most important part of SEO. Aprt from using all these tools. think as a user for the theme of your site, let us say your site is provinding software services, and being a user what all the keywords you may type in search engines to get software services, are the best keywords, if your site's theme is competitive and your site is new then better to target less competitive keywords to get start. you can also use word tracker KEI. "Only one keyword can change the life"