When powering it up it starts to boot I get a blue screen then it goes black where do I start ?

Try starting your mac from it's install disc.
1) First turn off your computer.
2) Then restart it, right when you push the power button to turn it on, press the option key down and hold it
3) pressing the option key will bring to a screen where you can choose your startup disk. Usually, you'll see your Macintosh HD available to startup from. Don't choose anything yet.
4) Insert your install disc into the CD slot.
5) wait a couple of seconds and then clock on your install disc as your startup option.
6) after clicking on the disc, go to the menubar and click on Utilities then disk utility.
7)once the disk utility app is open hit "verify disk permissions"
8) after you've "verified disk permission" hit "repair disk permissions"
- this might take a long time depending how much you've actually done this (which it's good to do every other month or so to keep your mac running smoothly). things might seem to freeze up as well, but it's the mac trying to verify and repair broken permissions on your computer. Don't that's normal.
9) after you have repaired the disk permission, reboot your mac by quitting disk utility and then choosing the startup disk as your normal macintosh HD. That should do the trick. If not, tell us what has changed.