What is Iago's motivation in Othello?

That is such a good question, because there are so many ways to answer it! Iago is a classic example of the 'Machiavellian' villain, something found all the time in works from Shakespeare's era. So, there's your first point: he's familiar to the audience. They are used to having a villain be villainous purely for the fun of it. (If you want to seem extra smart, you can always raise the point here that Shakespeare was known for giving psychological reasoning to his villains, thus making Iago a curious exception!)

But what can we see if we look at Iago's scenes? Predominantly, there's racism, and this comes to play in two parts. Iago is very jealous of Othello's position, constantly spouting how he should be the one in power (I won't list the quotes for you, you can do that yourself), but he is also insistent that Othello should not have a white wife, Desdemona. This is where his animalistic metaphors come into play, such as one of his most infamous phrases, 'the beast with two backs'. In an era where people of colour were seen predominantly as lesser races, both morally and intelligently (note: contextual references will get you good points), comparing Othello to an animal is highly suggestive as to his race's lesser position in society: something which would hurt Iago, as his subordinate. Simply: he wants Othello out of the way so that the hierarchy of the white people can come back. (Sidenote: don't be an Iago, guys. Everyone is equal.)

This is the most common understanding. There is plenty of evidence for racial strife from so many characters in the play. Even Othello himself, towards the end, hints his belief that Desdemona was too 'pure' for him (ie, two white for his blackness). You can even see Shakespeare as being racially prejudiced by having Othello falling so quickly for the tricks of the 'green-eyed monster' and so swift to go insane. But if you want to show off your intelligence, don't just argue this one theory. Absolutely reference others, and draw on critics to do so. Coleridge famously calls Iago's evil doings the 'motive-hunting of motiveless malignity', so you're in good company if you want to argue that direction. And hey, be experimental: people have even pointed out that the end of Act 3 Scene 3 resembles a marriage between Othello and Iago - 'I accept thy love' and 'I am your own forever', anyone? - and used it to argue that Iago is jealous due to his buried homoerotic tendencies (re: Shakespeare's friend, Christopher Marlowe, if you're interested in this aspect of Shakespeare's works).

Basically, there is nothing stopping you go wild when you're faced with this question. The only thing you must make sure you do is back it up with facts. Point, explain, evidence will always be your friend. You can argue that Iago is in love with Othello and is taking the 'if I can't have him no one can' route, but only if you can back it up with facts. Read and re-read the text, and have fun coming up with theories.