There is no easy way to build good nginx configurations by translating your Apache configurations. I did find a site that offers a translator, but it often generates configs using the If directive, which can have some nasty side effects inside nginx.
The nginx wiki is a good technical reference on what nginx can do and how to set things up. It also has examples of full configuration files.
Most people add rewrite rules to their modproxy configurations, and nginx does have URL rewriting rules, but they work differently from Apache’s and nginx also provides more efficient ways of handling some situations. Please read the Common Configuration Pitfalls for examples of that.
