How can i apply for visa? how much time it takes to apply? and is it difficult to get one?

Aijan, first you should make an appointment for the application throught the official website of CVASC London. In the meantime, prepare your passport, a photocopy of the passport's info page, a properly filled-out application form attached with your recently taken half-length color photo, a declartion downloaded from the website and with your signature, the original and photocopy of your student certificate, and an invitation letter from your friend. Then go to the CVASC office or send your application materials there.

The normal process takes four working days. As you are not a native in London, maybe it will not easy as it's for the locals.