Well… you have an older model which doesn’t offer onboard OS recovery!
You need a local source for the OS files so the system can boot up and offer the needed version of OS-X
At this point you need to seek out a friend with a Mac who can help you create a bootable external drive (a USB thumb drive will do)
You will need to figure out which model you have to know what OS-X your system needs. Plug in your systems S/N here [https://everymac.com/ultimate-mac-lookup/|System Lookup] then locate at the bottom of the listing for the OS info.
You may want to look at other OS’s your system could support. The reason is Apple doesn’t offer newer OS’s that will work with is model series and many Apps now require much more CPU, RAM & Storage than what this system can offer.