Frankly, this is not easy, and not for the faint of heart. Having someone to help who has done this will save you a ton of time and headaches.
Set up and use Back to My Mac - Apple Support
Before you do anything though, Back to My Mac assumes that you have a simple modem......not a modem/router that is often provided by Internet Service Providers to their customers. A modem/router.....also called a gateway.....will not allow Back to My Mac to work correctly, so if you do have a modem/router or gateway device, you will need to speak to your service provider to find out if they can provide you with a simple modem.
A simple modem will look something like this:
A modem/router or gateway will look something like this:
Having said this, personally I use another method called Port Mapping to allow my Mac...or an iPhone or iPad connect back to the Time Capsule at home when I am traveling. But, this is even more complicated than Back to My Mac, so BTMM would be the place to start for most users.
If things look intimidating on first pass and you can locate a good tech to set things up for you, that would be money well spent.