There are many aspects – technical, financial, regulatory – to running a registry. Reviewing the Applicant Guidebook from the 2012 application round is a good starting point. ICANN is currently updating the Applicant Guidebook and while certain details will change, it provides a comprehensive and current overview of what you'll need to know to apply for a new gTLD. You are invited to follow the work of the team that's developing the new Applicant Guidebook here.