Here is a list of free tools which could help you to do keyword research. You can look into these and pick 1-2 based on your preference. Using this in combination with adwords tool will give you very effective result.
Generates a visual and textual representation of related keywords
Quintura
Code:http://quintura.com/
Generates a visual tag cloud in addition to its own textual search results
Reference
Code:http://www.reference.com/
Generates dictionary, thesaurus, and reference information
Visual Thesaurus Generates a visual representation of word relationships
Code:http://www.visualthesaurus.com/
Glossarist
Code:http://glossarist.com/
Provides links to glossaries and dictionaries on all subjects
Webmaster Toolkit
Code:http://webmaster-toolkit.com/
Generates related keyword searches using many search engines, including Google
Keyword Discovery
Code:http://keyworddiscovery.com/search
Generates up to 100 related keywords and an estimate of their daily search volumes;
a paid version is available with no restrictions
Wordtracker
Code:http://freekeywords.wordtracker.com
Generates up to 100 related keywords and an estimate of their daily search volumes;
a paid version is available with no restrictions
Search Forecast
Provides a keyword suggestion tool with the (predicted) number of searchesCode:http://searchforecast.com/