When I turn the system on I only get a white screen
Can someone help me please
This is usually hard drive problem in my experience. Disconnect the hard drive/IR cable from the logic board. Reboot using your original system installation DVD. If you get a boot, the problem is isolated to the hard drive or HD/IR cable. One or both may need replacing. Let us know before you spend any money or proceed.
Please try reseting NVRAM: Shut down your Mac, press the power button, and as soon as you hear the startup chime, hold down Command-Option-P-R. Keep holding down those keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally. Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure they’re set the way you want them.
First off, make sure you don't have any external drives connected (flash drives, SD cards, etc).
This sometimes means that your OS has an error. Try holding Option when you start your computer. It might come up with the option for your main hard drive and/or a Recovery Partition. If it offers the main hard drive, select that. If it boots properly, make sure to set the main hard drive as your primary boot device. If the main hard drive is not an option, select the Recovery Partition. If that is not an option then restart and hold CMD + R and that will boot you into internet recovery (you'll need an active internet connection) and then you can recover from there.
I think you need to visit the Apple Store and let them fix this.