Let's start by trying to find doubts, and question reality...

If I am wrong about reality sometimes, maybe I am wrong all the time. But no, even if I think I see something, I can always check and correct myself, I couldn't be wrong all the time.

Maybe it's like I'm in a dream...no, even in dreams, some things are always true, for example, I am always in the dream, and two and two still make four, so those things are still true.

Ah ha! I've got it. Maybe I'm a brain floating in a jar and there's a mad scientist sending electrical signals into my brain, making me think I am seeing this and feeling things that I'm not. Suppose he is confusing me about everything, even the simplest truths that I believe. Suppose everything I think, including 2+2=4, is false, since I am being fooled by this evil sorceror / mad scientist and being made to think it's true when it isn't.

Let's try and get out of this hell now...

To find what's real, lets throw away anything we're not sure of, and make a clean, fresh foundation to build correct beliefs on. I don't know for sure that I'm typing this, that I go to school, that my friends are real, etc. Lets just throw all that out. Anything that can be questioned, just forget about it. What's left? Nothing?

There is one thing left: I exist. No matter how hard I try, I can't even question it. If I think, I must be, even if I'm wrong about everything, and nothing is what it seems, and I'm all confused, at least there's a ME to be confused.

Now, I (which I can't deny exists) have the idea of "perfect" in my mind. Something greater can't come from something lesser. The idea of perfect certainly didn't come from me since I am far from perfect, I don't even know if I'm typing this for goodness sakes! So...the idea of perfect must come from something that is perfect. So there is something out there that is perfect: all powerful, all knowing, all good, etc. The only one by definition to fit that description would be what we call "God".

So there is a God which has given me the idea of perfect, like someone putting a stamp on their creation. OK, now if he has the attributes of perfect, he must be all good. And one that is all good wouldn't allow me to be constantly confused in a hell of illusions. Therefore, the world is real. If I am wrong about some things, I can find that out and correct myself.