How can I configure my ADSL modem and Home PC to be able to access from external?

"To expand on the answer by TheTXI

Using a service like DynDNS you create a new account which will provide you with a unqiue Url, for example myhome.dyndnsdotorg.

Depending on whether you have an ADSL Router or Modem the process differs slightly. When using an ADSL router you need to configure the Dynamic IP settings on the router to point to your new DynDNS URL using the username and password you supplied. This will ensure that every time the line resets the new IP is given to DynDNS. This process normally takes about 20 minutes. If your using an ADSL Modem you will need to download seperate software to do this.

When using an ADSL router you also need to setup Port Forwarding. This will allow the router to send data for a particular port, either VNC or RDP to your machine. To use this I recommend either giving the machine you want to connect to a Static IP directly or using DHCP assigned static IP based on Mac address."

DynDNS has become paid now. No-ip.com offers the service for free. You can see a detailed tutorial to access your computer connected via a wired ADSL modem + wireless router from external network at himanshuforhpc.wordpress.com/

"save all the hassle, get a permanent IP from your ISP for very cheap
and set port forwarding or a demilitarized zone (DMZ) with your router."