Cool no worries.
First thing IMO to identify whether you can get a console on the device, i.e. whether you can get a shell? (Serial console / ssh / exploit etc).
I would then start by trying to get a dump of the existing flash etc so you have a backup.
In parallel, you will probably need to learn how to build firmware. (i.e. set up a build system).
Here's the wiki links I've previously collected:
https://openwrt.org/docs/guide-developer/hw.hacking.first.steps
https://openwrt.org/docs/guide-developer/adding_new_device
https://openwrt.org/docs/guide-developer/add.new.device
https://openwrt.org/docs/guide-developer/defining-firmware-partitions
https://openwrt.org/docs/guide-developer/device-support-policies
https://openwrt.org/docs/guide-developer/toolchain/use-buildsystem