First thing you need to do is make sure you can perform an Option Boot in order to access the Apple boot picker menu. Hold down the Option key immediately after hearing the startup chime which should give you the Apple boot picker menu. It is possible you won't see any valid options on screen, but you should be paused at that screen regardless. I forget what the Option Boot screen (aka Apple boot picker menu screen) looks like for a Mac that old if no bootable volumes are detected. If you are not able to access the Option Boot/Apple boot picker menu screen, then try using an external keyboard (on a Windows keyboard Alt is equivalent to the Option key). If still no luck, then try a PRAM Reset (hold it for at least three chimes if possible).
If you cannot perform a PRAM Reset or an Option Boot, then you probably have some sort of hardware issue with the laptop. Theoretically if you have a valid bootable CD/DVD or USB stick/drive with a compatible OS, then that CD/DVD or USB drive should automatically boot assuming no hardware issues with the laptop.
Because making a bootable macOS USB installer is so difficult if you don't have access to another compatible Mac, you can instead create a bootable Linux Mint USB installer to test the laptop. Whether you just confirm you are able to boot the Linux Mint installer or actually install Linux Mint to the internal drive does not matter since we just need to confirm you can boot to something. Of course installing Linux Mint onto the internal drive is the best test since then you can confirm the laptop is operating with a full OS from the internal drive. I'm not saying you need to keep it installed, but it would be a good test here. There is an option with the Linux Mint installer to boot into the Linux Mint OS (called "Live" mode) so you can see what the OS looks like which can be good test here is you did not want to perform an installation.
You can create a bootable Linux USB stick by using the downloaded Linux Mint ISO file as a source for Etcher (Mac, Windows, Linux). To boot a Linux Mint USB installer, you will need to do an Option Boot, then select the orange icon labeled "EFI" on the Option Boot/Apple boot picker menu.
Otherwise your options for installing macOS are limited to either creating & using a bootable macOS 10.11 El Capitan USB installer, or by booting to an OS X 10.6.x Snow Leopard upgraded DVD installer (then upgrading to macOS 10.11). You should have this Snow Leopard DVD if you ever upgraded macOS on this laptop past the original OS X 10.5.x.
In order to create a bootable macOS 10.11 USB installer, you will need access to another compatible Mac generally from 2007 to 2015. You can check the following article to confirm which model Macs are compatible with macOS 10.11:
https://eshop.macsales.com/guides/Mac_OS_X_Compatibility
Here is an Apple article with instructions for creating a bootable macOS USB installer (within the article is a link to download the El Capitan installer from outside the App Store):
Create a bootable installer for macOS - Apple Support
