First of all joining the SE3 Ladder is joining a community. You should plan to be an active participant and not join "because it looks neat" or to cause trouble. There are instances of people joining and setting up an empire on the Ladder and then never to heard or seen from again after a couple weeks. This action disrupts the community since "dead" empires are not preffered members.
First thing to do before seriously joining the Ladder is to read the Ladder Rules. This will tell you how the Ladder system works and will give you an idea of what is expected of being a ladder member. You can follow this link to Ladder Rules re-posted on this site here.
Many of the Se3 Ladder members own the fully registered version of Space Empires III so unregistered owners should purchase the complete version from Malfador Machinations. Since this will provide all members with the best gameplay.
You can only have one empire join the ladder since having multiple empires is considered cheating on the Ladder. Think of a unique empire name and provide some history of your empire and people so you can post it on the Ladder website. Having knowledge of HTML and the use of graphics editors (like Paint Shop Pro) will allow you to even post your empire's emblem and a picture of your species.
Another factor to think about is what kind of empire or species will you be? Agressive, peaceful, nuetral, traders, mercenaries, and so on....its up to your imagination and creativity.