The version you just installed is incompatible with most
of the Giants servers being played today. You need to update Giants to version
1.4 before you can join these games. Most importantly, here's
a link to that patch.
To play custom maps, you need a program called Giants
Mod Manager (GMM). Once you've installed GMM, you can either join games
and download the map off the host, or you can go to the
map database to download and host maps on your own.
Making your own maps is a very tricky process and requires
many different tools. There are a couple guides to performing this process,
all of which you can find in the tutorials section.
Well, I'm usually a good sport for any type of data, as
long as it's Giants related. First, you need an account on GiantsWD. Once you have an account and are logged in, you will see an "upload" link on the main page.