Here’s a tool I use:
http://www.anilcetin.com/convert-apache-htaccess-to-nginx/
It is not 100% accurate but it’s pretty good base
Also, here’s a link about converting the rules:
http://nginx.org/en/docs/http/converting_rewrite_rules.html
This one can help a little:
http://wiki.nginx.org/HttpRewriteModule#rewrite
EDIT:
The file name should be nginx.conf