Make sure you have all your drivers up to date for your computer's hardware and software. (this is usually taken care of by windows or mac osx automatic updates however attempt a manual automatic update to make sure)
Ensure you have the latest firmware installed on your router. This can be a little tricky if you are not technically savvy but it is a fairly simple process to follow once you find the right guide. The best bet is to google search for your router's brand, model number (usually found on the device) and "firmware update". Or better yet, find a computer savvy friend to help you.
A good place to look for help in this regard is Whirlpool.net.au:
Be sure to print or write down the steps and back up your router settings before venturing on this journey. If you are not confident enough to do this then perhaps leave it for now and seek further support from a friend or your ISP.
Blizzard have been kind enough to run through the steps necessary to optimise the most common firewalls on computers.
You can find their article here:
The general idea is to do the same steps you performed on your firewall to your router. Usually in the form of setting up a virtual server for the ports and for each Computer IP address on your network.
Now the reason I assume Blizzard has not supported a more extensive step by step guide for this is because there is such a vast number of routers available with different firmware software installed that it would be nigh on impossible to cover them all in detail and beyond their scope. Fair enough I guess.
That said, I did a bit of searching around and found a great place which covers the gap Blizzard has left, it helps you configure your specific hardware for Starcraft II specifically:
They have a great step by step guide for setting up Starcraft II specifically:
From my viewing of a couple of random guides it seems accurate and safe. All you need to do is select your router model from the list and then skip the ad on the next page before you reach a step by step guide on how to forward the ports for your router in regards to SC2.
UPnP being set to disabled is known to effect voice communications.
Ensure you have universal plug and play enabled on your router. This is usually enabled by default. Look for "Universal Plug and Play (UPnP)" in your router settings (sometimes under device settings) and make sure it is set to "enable". Standard port is 2800 though only try changing this if you are having issues.
After you complete those three steps you should be ready to play SCII without any network difficulties from your end of the line.
If people get stuck on certain points or have suggestions to add more reference material please feel free to post here and I'll help where I can and / or update the guide.