How do I make sure that my essays have a good introduction and conclusion?

In my opinion it's best to write your introduction and conclusion last, but this may not be easy in a written exam (in which case the best thing to do is either leave ample space for your intro or spend extra time planning so that you have as clear an idea as possible where your argument is going). Either way your intro should primarily be doing three things. It should give an idea of what your overall argument is (i.e. the main point of the whole essay) and also outline how you'll go about making this argument (i.e. a quick breakdown of the paragraphs that you'll be writing). Finally your intro should efficiently introduce the topic. I think opening with a general but profound statement about the work or works being discussed is a strong way to start your essay. For example: "The Iliad was most likely composed by various poets over many generations, rather than any individual 'Homer', but this does not mean that the work lacks coherent themes and motifs..." (this could lead on to an essay about the theme of suffering in the Iliad, for example.)The Conclusion should primarily summarise the points that have been made in the essay, and tie together your argument. The summarising here should be fuller than the quick breakdown of the introduction, but being concise is key - you don’t want to simply end up repeating yourself. As with the intro, it is best to end your Conclusion with a 'bombshell statement' which eloquently summarises your argument (the kind of sentence that makes you want to 'mic drop' your pen after you write it).