OS won't startup? need some help please :)

2007 OS X**
Okay, this is my first post, and the first time I've ever posted on any kind of forum!
To start off, I'm pretty useless with Mac's, this is my first one after years and years of using Windows computers and laptops, so some non-sarcastic, simple (if possible!) help would be good.

It starts up as usual with the apple logo and the sound, but then the grey screen stay on and a circle with a line through it come up, then the apple logo comes back and then a flashing folder icon with a question mark in it appears.

How can I reboot or troubleshoot it? Unfortunately, I don't have the system disk or anything and I can't find anything else online.

The Operating System is damaged and needs to be either repaired or reloaded.

To elaborate.... there is no OS to be found, be it a failed HD, or extremely corrupted OS.
Since you say 2007 OS X, this leads me to believe that you have an intel Mac, and your best bet is to go to Apple, Best Buy, etc, and purchase the $29 Snow Leopard install disc.
http://store.apple.com/us/product/MC573Z/A?fnode=MTY1NDAzOA&mco=MTc1ODgwMjE
Then, you have options!

Boot from your install disc holding the C key as soon as you hear the chime.
Release that C key as soon as you see the spinning gear.
Select the language when it comes up, and click next and wait until the menu bar appears.
Select utilities, disk utility, and select the HD at the top of the left column and then select repair disk.

If the HD doesn't show up, then your HD is toast and you need a new HD.