First of all, forget the VPN for a moment and make sure you can get a dial-up connection to the central network and that you can access everything over that connection that you need.
Once that is working, set up the VPN client software on your remote PC and the VPN software at the centre, making sure you have all the options set correctly. Then see if you can connect.
Your VPN probably won't work first time. Then go through and find out which settings are set incorrectly until you get it working. This is where you will want to ask for more help!
Configuring VPN services for your router is quite a simple process because the 1721 supports the Cisco Security Device Manager (SDM) which allows you to configure 'easy vpn' with just a few clicks through a web browser interface.
You will need to enable the http server of your router (ip http server) in global configuration mode and then access your router's private IP address from a workstation's web browser.
From there its a 'follow the bouncing ball' process
If you require step-by-step help to get the http server going, let us know.