A) SENTENCE TRANSFORMATION is the first part of WRITING in B1 examinations. It contains 5 questions in which you have to re-write a sentence so that both sentences have the same meaning.
B) WRITING AN INFORMAL LETTER OR EMAIL is the 2nd part of Paper 1. You'll have to write between 35-45 words.
C) WRITING A LONGER PIECE is the 3rd and last part of WRITING in Paper 1. In this part you'll have to write an answer to one of two questions. You have to write about 100 words.
Here you have more links to practice this part of B1 examination.