Well, there are certainly plenty of themes you could talk about, however, I am just going to focus on one. Hopefully that will give you some ideas of how to approach other themes in the novel and write an essay about them. If you need more help I have added a link to the enotes section on themes for this novel below.
To think about friendship and how it is presented is to think about the tragedy of the novel. The memory of the tragic friendship between George and Lennie lingers on long after the close of the novel because the friends have lost a hope or a dream that is bigger than their friendship. Consider how the dream of the farm operates in this novel, and how it compels and sustains not only George and Lennie through difficult times, but how it also attracts like a magnet other lonely characters. Consider how Crooks is drawn to the idea and likewise Candy. Although Crooks is cynical and has seen many other men with the same dream, he still is seduced by it and wants "in" on this dream with Lennie and George. Friendship, therefore, centred on this "dream", shows how the men in this novel crave companionship and fellowship that would defeat the loneliness that rules their lives. To have someone that they can share life with means that they have someone who will protect them and look out for them in a very harsh and brutal world.
However, and very depressingly, the world that Steinbeck paints is too brutal and too harsh to form and continue such an idealised friendship. This is of course symbolised tragically in how the friendship between Lennie and George ends. This tragedy is made all the more poignant by the repeated reference to their dream of living in harmony which is given enough credibility to make it achievable. However, with Lennie´s death, a true example of friendship and brotherhood ends, but what makes it more tragic is the lack of understanding that some of the characters have when they come upon George and Lennie - they do not recognise the world-ending significance of what has happened to George. Not only is such a friendship impossible in our brutal world, Steinbeck seems to be saying, but others are unable to recognise it even when it does occur.
Hope this helps! Now you might want to try analysing some of the other themes like the American Dream, discrimination and Utopia. Good luck!