Default username and password vary by router manufacturer and model.
Download the manual if you don't have one to find the defaults. You'll have to do a Internet search for your specific model and include the terms "manual" or "setup" in your search.
You'll need the administrative password to make any changes to the router configuration.
The wireless network name (SSID) and WiFi password used for wireless security is not the same as the administrative username and password used to access the router's configuration interface.
Passwords in the router user manual or printed on the back or underside of wireless routers are the factory defaults. For security these passwords are changed (or should be changed) when the router is first configured.
If you don't know the administrative login password, you'll have to reset the router to factory default state. The reset button is usually behind a small hole on the back of the router. To reset the router, use a toothpick or similar object to depress the button for several seconds while the router is powered up. If the router is reset to factory default, thus erasing all user modified settings then it will revert to the default passwords and settings. Be sure to change the default password of the administrative interface of your router for security.
Here is how it port forwarding works,
Port forwarding is used to allow outside access to specific devices on your network such as security cameras, web and gaming servers, etc.
Here is some good info on the subject from Steve Gibson. https://www.grc.com/su-ports.htm
Every time you open a port, you put a little hole in your firewall that can be accessed by a outside system.
If you forward that port to a server then any security vulnerabilities in the software running on that server could potentially be exploited.
If the destination of the port forward is secure then there are no problems.
When you are connected to the Internet, you actually have two different IP addresses, a private LAN IP and a public WAN IP.
In most home network applications the router connects your local group of computers and devices known as the LAN with the Internet, commonly called the WAN or Wide Area Network.
Unique local LAN IP addresses are usually automatically assigned by the DHCP service running on your router, but can be set manually (static). They are in binary but usually expressed in readable format such as 192.168.1.1. The IP addresses are private addresses http://en.wikipedia.org/wiki/Private_network and are not routable across the Internet.
Your cable or DSL modem will get a Public WAN IP address from your ISP. These public IP addresses are leased in blocks by your ISP and are Internet routable. The DSL or cable modem is connected to the WAN port on your router. The router will manage the traffic between the devices on the LAN and the Internet.
So, your LAN connected devices will all have unique private IP addresses assigned by the router and share the WAN IP connection via the router. No one else can have the same WAN IP as you at the same time or the Internet servers would not know where to send the data packets.
The only IP address that a website or outside device can see is your public WAN IP. They can't see the private IP addresses on the LAN side of the router.
You can see your current public WAN IP address here: http://www.mywanip.com/
Depending on your ISP your public WAN IP address might be dynamic (changing periodically) or static (fixed).
If you have a dynamic WAN IP and need a fixed IP you'll have to contact your ISP or use a DDNS (Dynamic DNS) service. http://en.wikipedia.org/wiki/Dynamic_DNS
Good Luck...