Any Ideas?

So, I've been thinking on this story I'm trying to write for the contest. I am having trouble with it though because I don't know where to begin. I've researched the publishers and found that they tend to publish either contemporary fiction or historical fiction. I'm not sure which genre I want to go to because there are pros and cons to both. The hardest thing for both is going to be research. Can I do enough thorough research to write a story in around 150 days? And then there is the actual story, involving plot and characters, etc. I have some ideas but I'm trying to find something that clicks. Should I use an idea I already have or should I start on something completely new? So many questions and so little time to answer, especially considering I have to write it after figuring these things out. What am I going to do? Any ideas?