What can I say about these accessories. You absolutely need them, but which one you acquire is mostly a personal preference or depends on how much money you want to spend. There are not a lot of technical details you need to be concerned about. You'll need to get one of each of these for your computer. I will add some thoughts you might want to consider.
On the monitor, get the biggest size that stays within your budget. LCD monitors are the current best choice. As HDTV's are starting to come with computer monitor capability, so computer monitors are starting to come with built in digital tuners and HDTV capability. I expect the two to merge. Your buying decision will then come down to how big a monitor you can afford. Most of the other specs are not that important. There are several good manufacturers of monitors.
On mice, I recommend a wired optical mouse over a wireless mouse. I find that there is a lag in the wireless mice before they wake up. You have to shake them a little to wake them up, while the wired mice always seem alive the instant you touch the mouse. What that means to you is how responsive the mouse feels. I personally want a very responsive mouse, especially for computer games. Wireless mice can be very frustrating at times. If your farther away from the computer than the standard 6 ft. wire on a mouse, a USB extender cable can be used to extend the distance.
The keyboard is another story. I prefer a wireless keyboard. over a wired one. When your typing, you don't need the quick response of a mouse. I have had no problem with wireless keyboards.
Beyond this both mice and keyboards come with various features to make your computer use easier. It is mostly a personal choice as to which one you purchase. I personally prefer Logictech mice and keyboards over other manufacturers.
Speakers are a user specific, personal choice. If your an audio surround-sound affection-ado, you'll have a surround sound receiver with a bunch of speakers, if you just want sound out of the computer, you'll just have a couple of computer speakers. The choice is yours. Today's computers can output some very sophisticated digital sound, which you will then plug into the speaker or receiver of your choice to hear.