I would suggest using Google Trends and Google insights to find what topics are hot... Then use Google's external Adwords keyword tool to find out the US and global estimated monthly search volumes for related keyword phrases... and then use Google to see how competive each of those phrases are. You can do all of your keyword analysis free.
You may find some good tools by google like the adwords keyword tool, google analytics etc...but I have a suggestion. Use the adwords keyword tool to find out the cost and relevant keywords. If possible run an adwords programme with low CPC. Since the CPC will be low there are less chances that people may click on the ads, so you cost will be low but at the same time you will get the data of the keywords for which there was maximum impressions. The adwords programme is one of the most accurate way to find the relevant keywords.