Use the web. It is great for research. Google yourself and you might be surprised what you find. Or use Bing another great search engine. There are so many free resources out there!
You now will want to visit the affiliate program you signed up to and look for products in your niche to advertise. Now pick one specific product, read their entire sales page and write an article about this product and you will then take that article and put it on your brand new blog. Yet another thing you will want to do is to be sure that somewhere in the article or even at the end of the article is your affiliate link. Having your affiliate link placed in the article is the only way men and women will be able to find the product. Any time these consumers decide to buy the product you earn a commission.
The plus side of the dot ORG option is that you have total control. You can maintain your blog system yourself, back it up, download a copy for safekeeping and easily install any extras you want (plugins, themes etc) using FTP. You also benefit from it being called whatever you choose. You can migrate it from one host to another and keep the domain should the need arise.