I know all my friends will suggest Google adword but any one can explain adword..The keywords selection is not as simple its also required some Good knowledge about SE queries for words you are selecting..For example one of most expensive word is "web design"alot of searches are for this word..but at the same time a lot of competitors too for same word..Now less expensive word is "Affordable website Design" less competitors but less searches..Mean look both visitors and searches
Read this if you'd like to get an idea about free keyword analysis using the Google external Adwords keyword tool.
I use google adword for keyword search and i recommened that tool to u. Its really nice & easy in use.