You have two options.
From installation point easier, but harder to configure is the openvpn. It is part of the Debian distribution, just install with apt-get install openvpn. With openvpn, the authentication is based with keys, not with username and password, you have to create keys and share it with the clients. The advantage, you have client for Windows and Linux as well. disadvantage, you have to configure the client and the server as well, and you need the key for set up a new client. I don't say it is easy to configure, but you can find very good howtos on the Internet.
The easiest howto is: http://wiki.debian.org/HowTo/openvpn
You second option could be the PPTPD. This is what debian used in the past, they switched later to openvpn. It is a PPP based VPN, easy to configure, because use the same protocol what Windows use, if your client is Windows, you don't have to set up anything on client side, just set up a VPN network in windows. It uses username and password, easy to set up a new client. Configuration on the server side is not a big deal as well. I just set up one some days ago from source code, installation and configuration was 3 minutes, everything is well documented in the source code package, for debian configuration files are well prepared.