it's a little complicated without knowing specifics. This is all described in the documentation and covered in the training. I'm going to assume you didn't take the training
For S-G, MBG with two nic's, you could replace your current internet f/w (the SP router?) with MBG itself. It does NAT and routing and was originally designed to be a small office internet gateway. Configure the MBG with the same LAN and WAN ip addresses as the existing f/w when you replace it.
Alternatively, you can put the MBG "astride" the current f/w. In that setup, you'll need to get a second static IP address from your ISP if you haven't one already. MBG will have a direct internet connection (along with your f/w, usually a switch sits in front of both so they can share the same drop from your ISP). And MBG will also have a connection to the LAN (with some address, not the one the f/w uses). That configuration is used quite often.
The third configuration requires you to configure your existing firewall with a DMZ, but IT HAS TO HAVE 3 ports... You cannot do this with a 2 port f/w, one-way audio will result.. Again, you'll need a second static IP for MBG, which you'll have a rule in the f/w to forward to MBG in the DMZ.
If the above statements aren't clear to you, then you need to get some IT help.
