You can get a two (real) button mouse with scroll wheel at any office or computer store.
I recommend the Logitech wireless laser one that has a tiny USB plug you leave in the machine all the time, the battery in the mouse itself (included) lasts 18 months.
A alias of a file allows you to place it in the Dock or elsewhere without moving the file from the original location.
It's useful of your burning files from many locations and then don't want to put them all back, you can just delete the aliases.
There should be a video camera embeded at the top of the screen, you can use PhotoBooth in the Applications /Utiities folder to take a video or a picture.
If you want to take a screen shot, then hold Apple/command shift 3 or 4 depending, there are other commands for windows and such too.
I have no idea about your last question
Your iMac came with a multi-button mouse. If you got the Magic Mouse, see:
If you got the Mighty Mouse, the one with the scroll ball embedeed, see:
An "alias" is the equlvalent to a "Shortcut" in Windows.
The camera is ready to go. You can, as DS said, used Photo Booth to take pictures of yourself, Facetime to do video chat with other Mac or iOS users, or other applications that support video such as Skype.
For more information about Spaces (which may be what you mean by "multiple windows" or "multiple desktops"), see:
No, my iMac 27" doesn't have an option for the mouse to be a 2 button one. I have found that different Macs work differently, to the point of even having different keyboard shortcuts. I have both a wired & a wireless keyboard and they each use different shortcuts, I can see from Aple support that the regular Macs do let you set a prefererence for a 2 click mouse, but mine doesn not. There is no such option under Appe/Preference/Mouse.
Thanks for trying to help.
I have a 27" iMac as well, and the option to program the buttons is right in the Mouse system preferences. For a Mighty Mouse (the one with the trackball):
This will be the same on all Macs; there's nothing special about your model of iMac that would make this different. If you don't see that, or something similar for a Magic Mouse, then either you have a defective mouse, you're using an old one-button mouse or a third-party mouse and not the one that came with your Mac, or something's amiss with your Mac's drivers.