First of all, let's look at the facts:
Manifest Destiny was an ideology of the 19th Century; holding that American settlers were destined to expand across the continent. Historian Robert J. Miller argues that there are three basic themes to it: the belief in the special virtues of the Americans and their institutions; America's mission to redeem and remake the world in its image and a divine destiny under God's direction to do so. (As you can see, the ideology has very deeply religious undertones).
Now, there are several ways of approaching the subject. One idea would be to write about how the religious beliefs of Americans in and before the 19th Century led to the ideology of manifest destiny; you could approach it by discussing the Puritan movement and the history of Evangelical protestantism.
Also relate Manifest Destiny to romantic nationalism ad the doctrine of American Exceptionalism.You could also look at how manifest destiny has been interlocked with racism, and how it may draw from the ideologies that justified the enslavement of the blacks and the expulsion and possible extermination of the Natives.
Alternatively, you could look at the economic situation of the United States in the early 19th Century, and see how expansionism would be justified economically; if you choose to make the point that economic situation shapes ideology, rather than the other way around.
You will need to look at the historical implications of manifest destiny. You can look at how the 'manifest destiny rhetoric has been used by the Democrats in the 1840s to justify the war with Mexico; as well as in the Oregon division case. How was ideology used to support certain political and military actions? How were these justified in politicians' speeches?
You could write about the controversy around it: historian Daniel Walker Howe argues that most Democrats supported Manifest Destiny, as they believed that the United States was destined to serve as a virtuous example to the rest of the world and most Whigs opposed it, on the argument that expansion would cause the extension of slavery to occupied territories.John Quincy Adams was formerly a supporter of Manifest Destiny, but later on repudiated it.
Another interesting thing you could do is to assess if and how the ideology, despite being grounded in the 19th Century, might still be found in today's political rhetoric. Think, for example, about the War on Terror or about how contemporary politicians use the idea of spreading democracy in order to justify waging war against non-democratic countries.