Well different forums have different rules, for example, pornographic images are not allowed here but would be allowed on a porn forum (most obvious example I could think of). As you spend more time here you'll learn what's right and wrong here.
First the more obvious things, make sure that the server isn't set to LAN and that the player count is set to more than 1. Make sure you're both running the same version of ESF. If you both have firewalls then make sure that your firewalls will allow ESF to access the internet. If you are behind a router then if you know what you're doing try setting up port forwarding for ESF/Steam. That's all I can think of.