I didn't intend my reply to demean you or your abilities, just to make sure you realize that this is a much larger undertaking than you may be expecting, with no guarantee of success. This is especially true if you have zero experience of networking systems... I'd generally recommend something easier to cut your teeth on.
The fact remains that while Mac OS X Server can do this, there may be easier/better solutions available. For example, if I were looking to setup a VPN connection to my home network (and I've been doing networking for a long time now), I'd look for a router that has a VPN server built-in... a few clicks in the router's UI and you're set. That's sufficient for most users' needs.
However, if you do want to go this route, your basic path is:
1) Enable the VPN service on your server. Here's a decent walkthrough - slightly outdated but still relevant.
2) Configure port forwarding on your router to map the incoming traffic to your server. The mechanics of this will vary depending on the router model in use, but check TCP and UDP ports used by Apple software products - Apple Support for a list of relevant port numbers.
Realize that being connected to the VPN does connect you to the internal resources on your home network, but it's still not quite like being there in person. You'll need to know the IP address of the various services (e.g. file server) on your home network in order to connect to it via VPN.