Go to system preferences and then go into network, on the first page you will see ip address. It will look something like this (192.168.15.103). Copy your ip and paste it in the address bar at the top of your browser (it will also work for mac, but for mac it wont work in google chrome, it has to be safari). For most of the routers, it will say
enter user name=admin and password=admin, this is used for linksys, type admin dor both. Nothing els. go into applications and gaming click port range forwarding. for start~end port put 25565. for Protocol put both for to ip address put in your last numbers then click Enabled, then click save settings

to find your real ip. go to www.ipchicken.com it will tell your ip address for the server

Thats pretty much it. If you have any Qs ask me.