We can help you up to a point, but we're not going to do your homework for you...;-)

The most obvious place to start is the idea of dualism; make sure you understand the distinction between substance dualism and property dualism as Descartes is generally understood as being one, but not the other.

If you have access to a decent library check out the first two chapters of John Searle's Mind: A Brief Introduction if you don't have the time to read Descartes directly (though I always try to recommend reading original texts if possible).

You need to be more specific than that. What is it you do not understand?

1 Premise: X is clear and distinct.
2 Therefore, X is a substance.
3 Note: Conceivably substances can exist on their own without aid of other substances.
4 Therefore, both God and X are substances. (Err??)
5 Btw, God is really sweet, so God gives X its substancehood.

For Descarte, substances are categories that "clearly and distinctly" and neatly encapsulate and logically found (without aid of any other substance) certain phenomena. Stuff like thought, believing, doubting, considering, hoping, reflecting, remembering, fearing, are all metaphysically possible without physical substance. He claims that if you can clearly and distinctly conceive of a thing, then it can exist without aid of another substance (except God, of course).

He claims that what cannot be conceived as not existing is thought. So, thought necessarily exists, since to doubt just is a mode of thinking, which just indirectly affirms the mode of thinking as such. He concludes that one must be physical in some way or other, but not necessarily as the way one perceives oneself. So extension gets to be a substance, for it necessarily exists (things must be one way or other), though that which is extended is contingent (like Descarte's handsome moustache). He claims to get to this conclusion by doing the mental legwork of "clearly and distinctly perceiving".

What a champ.

Look at it this way: (1) If I think, then I exist. Run the contraposition.

Suppose you do not exist:

(2) If I do not exist, then I do not think.

But that's crazy! Says Descartes, because surely to suppose that one is not thinking (to doubt that one thinks) just is to think (surely something must be doing the thinking to facilitate the doubting).

Crrrrazzy!

But the contrapositive of if I think, then I exist, is, if I do not exist, then I do not think, and it is not, if I do not exist, then I do not think, as you say it is. That would be wrong. But Descartes never says it, nor does he imply it. What makes you think he does? And neither does he say that existence is a sufficient condition for thinking, that is not true, since chairs exist, but chairs do not think. What he says is that existence is a necessary condition for thinking, and of course that is true, for as as I pointed out, the contrapositive of, If I think, then I exist, is If I don't exist, then I don't think, and not what you said it was.
And, of course, if something does not exist, then that something cannot do anything at all, and, therefore, cannot think.
This has nothing to do with doubting that one exists. Descartes does not say that if you doubt you exist, then you do not exist. That would be nonsense, since if you doubt you exist, then you exist. And, indeed, that is exactly what Descartes argues. He argues that if you doubt you exist, then you exist, for in order to doubt, you must exist.

I think you may be a little mixed up.

I'm going to stop responding to you because now I think you're just attacking me for the sheer fun of it, and making dopey mistakes while you're at it.

I am going to copy and paste your first sentence and provide line breaks just to show you how infuriating it is.

Now what do you expect me to say to this? Hm?

'Look, you're wrong. It isn't P, it's P. Okay? So you're wrong. Remember: "P", not "P".'

Wow.

---------- Post added at 09:58 AM ---------- Previous post was at 09:52 AM ----------

Where did I say the underlined? And if what I said, according to you, before the underlined, is "if I do not exist, then I do not think" then why would you attack "if you doubt you exist, then you do not exist"? So again, where did I say the underlined? WHERE?

Are you intentionally setting up straw-men? Why would you do that? These systematic mistakes of yours either make think you're just error-prone or you're being malicious. Neither of which I wish to deal with.

So I recommend, for better encountered with me, this:

Read (2) in my post again. Read my post and any post you see of mine a hundred times before you respond.

Never respond to any of my posts unless you've read them at least 100 times. That is a rule. If you do not, I will not respond to them.

---------- Post added at 11:31 AM ---------- Previous post was at 09:52 AM ----------

And even your own post is internally flawed. It's just a bunch of muddled thinking, a failure to proof-read and inconsistent, and failed use of, language.

The first line is a claim as to what Descartes does not argue. Then you call what he does not argue "nonsense"; but then you restate it. And then you say the restated thing, which is your restatement, (and which is not different from that which you call nonsense) is what Descartes argues, which is exactly what you said he does not argue in the first line.

Second recommendation for better encounters with me:

Read your own posts 100 times before you respond to any other post.

descartes is so kewl and like...

Just read Descartes, "Meditations." Easy book to read and follow.

That's a little optimistic, I think. Do you understand what Descartes means by a "real distinction" for instance? And why he thinks there is a "real distinction" between mind and body?

Yes, but he goes into it in great deal. And this is the book that started Modern Science. It is quite short. I suggest reading it or you will fail your test I can assure you. If you don't like reading don't take Philosophy my friend.