SonicWall Firewall + No Experience = PITA

No experience with Sonicwall appliances in particular but have experience with numerous other devices. But perhaps I can help you out a bit by steering you in the right direction.

The first thing you would want to do is create a network map. You need to know what type of devices are in your network and what they need to do. For instance if you have a internal mailserver and 12 workstations etc. Then You would want the mailserver (port 25) to be accessible from the outside world, if you have people working from home and need access to their email then you should decide if to either forward that port (143 imap, or 110 pop3, etc) or setup a VPN so external users can connect to your internal network.

Just take a peace of paper and start making some diagram, visualizing your network will also make it easier to understand.