It can be easy to get bogged down worrying about how to answer something like this in the "proper" way. Try to think, instead, of how to make it fun! After all, spending a million dollars would be fun, wouldn't it? :-)
Of course, how you approach the essay depends partly on what class your assignment is for. For an English class, you might get by with something whimsical:
"If I had a million dollars, before spending any of it, I'd get it all in $100 bills, throw the bills on my bed, lie down in the money and cover myself up in it. Only after becoming cozy with it would I be ready to spend my money."
For a class in economics, a different approach might be called for:
"If I had a million dollars to spend, I would start by making a list with two columns. The first column would be marked "Needs" and the second, "Wants." I would prioritize my needs and wants and see how far the money would stretch."
The most important thing to do with any writing assignment is to start writing! You can always go back and change it -- several times, if necessary! But start putting down words and see where they lead you!