First time questions.

I was just curious on how hard installing some new speakers would be. Is it easy to screw it up real bad, or is it pretty simple? And are there any websites you would recommend to first timers? I looked at some of the stickies for a link or two, but I didn't find any. Thanks.

I'm a noob and installing new speakers was pretty easy.

The ones I had you couldn't mess up on, they even came with instructions. And the hardest part for me was taking everything off to get to the speakers, after that it was pretty easy.

I learned a lil from this website http://www.crutchfieldadvisor.com/ISEO-rgbtcspd/learningcenter/car/speakers_install.html but it was so easy so I didn't use it much.

But you may want some advice from a more experienced person because like I said "I'm a noob"

install speakers is easy
the main think you have to look for is taking off panels to get to your speakers. I'm not good at explaining things but also on that site crutchfield also has a installation video which can be found here http://www.crutchfieldadvisor.com/S-Wv53JnIQwoc/learningcenter/car/videos/speakers.html

Another question. If I install new speakers, wil I also need to install an amp? How about a Head Unit? Finally, what kind of "box" is this: http://www.cardomain.com/ride/330249/2 ? Thanks again.

You don't NEED an amp unless you're going to have subs.
I don't know what kind of head unit you have but you only have to replace it if you want to upgrade, if its stock it may be a good idea.

So would I be able to use the stock wiring then if I'm only replacing the speakers in the doors?

Yep.
You should just be sure to hook up positive to positive and negative to negative.