Generally, it depends on the arrangement with the facility. Often you will be asked how you want the traffic exchanged. A typical question will be something like: "Are you planning to connect an L2 device or an L3 device?"
If the provider expects you to connect an L2 device, then the whole network will usually be bridged to you. You'll be given a gateway IP inside that subnet that should be the default gateway for your devices. This makes it difficult to subnet, but you don't need any routing equipment at all if you don't want to use it.
If the provider expects you to connect an L3 device, then there will usually be a small network (like a /30 or /29) that it outside the network they are routing to you. You'll be given an IP address for your end of that small network and they'll have an IP inside that network for their end. You then configure the link to them for that small subnet. They route all traffic to your main network to your side of the small subnet. You route all traffic for your provider to their side of that small subnet. This lets you subnet your main network however you please.
Most providers will first send you a sheet with specific questions about how you want the connection set up. After the provision it, they should give you a sheet with the specifics about how the link was provisioned to allow you to easily set up their end. (And to avoid having every single customer asking them the same 50 questions.)