Go with something you have first hand experience in - education, training, degree,,,,, something that would establish your blog as an authority in its field. One of the best blogs I ever read was about a doctor who worked in the emergency room, and the stuff that she saw.
There are many methods to find out that. The best of which is just google for " Adsense Keyword Tool" Play around with the tool and you will find that easy and a lot more.. Cheers Josh
There are many best keywords which lead to make money from google adsense but as you said you want to make blog - so i would suggest make a blog on the topic you know and have passion about - so that you can write something outstanding and when something interesting is written - so google will list it and user will read it - so you can make more money - than just following a trending keyword... hope this helps...
Check for the keywords on the Google Adword. There you'll find the trends of those keywords used for making money.
Use free keyword research tools by Google and Wordtracker. You will be able to find lots and lots of keywords.
I see two avenues. #1. Figure out what you know about or are passionate about and do your research on apporpriate keywords, domain names, etc to represent your subject. #2. Do the appropriate research on keywords that have the necessary traffic to determine whether or not you want to present a product to this market. Remember good ideas are dime a dozen, but this is a numbers game and numbers do not lie!