What networking should I learn before getting into cloud?

Start with general networking fundamentals, perhaps using a Network+ course or equivalent material. Then look at how virtual networks, subnets and routing work in your chosen cloud. You don't need to master every networking specialty before you begin.

For Azure, AZ-104 is a reasonable starting point. It covers networking alongside a lot of other administration topics, so it's useful if you want a broad cloud foundation rather than a networking-only course.

Pair the material with a small lab and test things as you go. Check pricing before creating resources, and clean them up when you're done. Stopping a VM doesn't necessarily remove every associated charge.

Give DNS some proper attention. Cloud setups can be confusing when a name resolves differently depending on where the request comes from. Once you've got the basics, look at hybrid networking, private DNS resolvers and conditional forwarding.

Routing is worth studying beyond just connecting subnets. BGP becomes relevant when you're looking at how company networks exchange routes with cloud networks, particularly in hybrid setups. It can wait until the basic routing concepts make sense.

For a practical exercise, look at how to restrict network access to Azure platform services. Compare the options and their costs rather than assuming every security feature is free. That gives you a useful reason to learn the networking pieces.