You will have to get your head around portforwarding one way or another!
It is simply a requirement to give people from the outside world access to specific devices on your local network, whether it be an ftp server or a NAS.
The only thing visible from your network on the "global" internet is your router/modem, and unless you tell your modem to which device to forward certain requests, it wouldn't know what to do with those incoming requests!
On top of this, with consumer internet access you'll likely get a dynamic public IP assigned, so you'll need to master the mechanics of Dynamic DNS services as well.
Although some NAS devices come with a preconfigured ddns service (Lacie springs to mind), but you'll still have to forward the proper ports to your NAS.
You'll have to start to do some research on the subject, unfortunately there is no "easy" software which can solve your requirements!
Doing what you want to do in a secure and practical way is unfortunately a tad above "basic" knowledge level, best to ask somebody with more knowledge about the subject to help you out.
It's a bit to complicated to write up a full tutorial on how to achieve your requirements