Your set text is Ted Hughes' The Birthday Letters. You have to know all of the poems just in case they specify one in the question, but they will usually only ask for at least 2 poems. For this topic, you need to choose at least 2 related texts which have something to do with Ted Hughes and Sylvia Plaths relationship - maybe one of Plaths poems, a Dido Merwin article, the movie 'Sylvia' etc. But make sure you understand the concept of the 'truth' and work out a thesis before you go out choosing your related texts.
Hope that helps