Read plaques about Arachnos patrons
Before you choose a patron you should get a better idea of what each of them has to offer you. Go and read each of the plaques underneath the main statues of Black Scorpion, Captain Mako, Ghost Widow, and Scirocco.
Once the decision is made it cannot be undone, so be sure to study the benefits of each very closely!
Mission Complete: You have leanred about the patrons.
Now you are armed with the knowledge of each of the patrons, you should be able to make your choice.
You should now know what each of the factions have to offer you, but if you are still uncertain you should go back to the patron plaques and read what they have to say. Remember: once you choose your patron, it will determine your allegiance within Arachnos from here on!
You must now choose which faction of Arachnos you wish to join.