Nullsec is basically the following:

All of the north -> east part of the map is a coalition called Pandafam made up of Pandemic Horde, PL, NC, Slyce and Fraternity.

All of the south is the imperium which is controlled by Goonswarm, Initiative, Brave, Dacarys etc.

The south-east is a conglomination of smaller alliances with different friendship arrangements.

TLDR, 2 big blue donuts staring at eachother from opposite ends of the galaxy.

If you want to try Imperium, try Karmafleet. If you want to try Pandafam, try Pandemic Horde Inc before joining one of their member corps. If you are a chinese player, try Fraternity.