If you don’t want to spend any money, emulating RISC OS with RPCEmu is a great place to start using RISC OS quickly and at no cost.
It is compatible with legacy RISC OS versions (3.xx, 4.xx, 6.xx) as well as RISC OS 5.
Although be aware that as brilliant as it is, RPCEmu is not fully representative of a complete RISC OS system – there are still some minor shortfalls that are not present in Virtual Acorn and native systems.
You will get a very good idea of the overall look and feel of the operating system, and it will be useable for running applications on a daily basis, but it can feel a little slow and clunky (especially when it comes to networking I find) when compared to running RISC OS natively on say, a Raspberry Pi.
There is also an older paid-for RISC OS emulator called Virtual Acorn – it is no longer actively developed but it is still available to purchase. While it is a very complete and stable application as a whole, it emulates 26-bit RISC OS and does not fully support RISC OS 5.