I'd suggest starting with the volume that picks up right where the anime concluded. It'll give you a seamless continuation and let you delve deeper into the story. Usually, you can find this information on fan forums or the official website for the light novel.
The ideal start for Overlord manga is the first chapter. It sets the stage and lays the foundation for everything that follows. It'll give you a clear understanding of the initial setup and help you get fully immersed in the story.
Another aspect to consider is time. If you have limited time, the anime might be more suitable as it can be consumed relatively quickly. But if you have the time and patience to read, the light novel will offer a more immersive experience. The light novel allows you to create your own mental images of the world and characters based on the detailed descriptions, which can be a very rewarding experience for those who love to read.
Yes, 'Overlord' is a light novel. It has an interesting premise where the protagonist gets trapped in a virtual world that he has to rule. It's been adapted into an anime as well.
The best place to start the Overlord manga after the anime is right after the last episode's events. You might need to do a bit of research to figure out the corresponding manga chapter precisely.