sharing my thoughts and suggest me if you can

Hi,
at first sorry for my English.
I want to start t-shirt business on teespring. I have good designing skills but I am confused about niche selection. If I don't select any niche and design most trendy t shirt ... Will I get sale??
at first I want to use free method of promoting.... So I need few suggestion about promoting my t shirt...
i hope I will get help here
thanks