If you need to apply for a six month Student Visitor Visa, please follow the steps below.

Go to https://www.gov.uk/study-visit-visa/apply, then click on the link 'apply online' (mentioned in the first sentence).

Select the first option "Short-term student visa" and click the green next button.

This should take you to the appropriate section of the application. 

More information about visas can be found on the Student Visitor Visa page of the UKBA website.