How Do I Get A Student Dependant Visa To The Uk?

I am currently studying in the Uk on a student visa, and want my wife and son to join me here very soon; i need advice on how to go about their application and what is needed for them to apply for a student dependant visa. They were both listed as my dependants when i applied for my own student visa. I understand there is a new application form in use at the UK embassy in nigeria but there seems to be no section for student dependant but i know theres a category of visa that covers dependants; please i need ur advice and help urgently. Thank you