You're not supposed to put the word "I" anywhere in a sociology essay, you're not arguing your own point of view you're arguing the sociologists'.
For the essay on postmodernity "Assess the view that religious beliefs and practices are changing to replace a new era of diversity and choice" I'd start by saying how in contemporary society, evidence has shown that religious belief has decreased etc through church attendance statisitcs, however some sociologists (postmodernists) believe that religion is not declining but changing into a more privatised form. then you could include authors such as:
Davie - believing without belonging/vicarious religion
Hervieu-Léger - spiritual shopping
Lyon - globalisation/growth of consumerism
In terms of ending an essay, if it's an "assess the usefulness.." question then you should decide which theory is more useful and state why it's better than the other theory in your conclusion, and throughout after making a point you should say "this shows that the theory is useful because..." or "this shows that the theory is not useful becuase.." this should get you more evaluative points!