For this i really think you need to do your research. Decide on what you like to do/talk about. Then start by using google adwords keyword tool and seeing if other share your passion. You can also use google trends. Once you have found you niche go and find your domain name and build a plan! The plan is so important.
It takes time and there are many topics out there, its hard but what isn't that's worth doing! Remember it always works better if you do something in the area of life that you love! Decide what you like then hone in on the niche or even micro niche.
Best of luck and remember to be patient.