You need a computer running one of the supported operating systems. As a general rule, the faster your computer is, the better.

If you want to compile the source code yourself, you need an ANSI C compiler.

Apart from that, you need a Kickstart ROM image file for most purposes. Software to transfer that from your Amiga to your PC is included in the source distribution, as well as documentation on how to use it.

Please note: The Kickstart ROM is copyright 1982-1997 Amiga Corporation (formerly Hi-Toro) and its successors and licensors and we are not allowed to distribute it here.
Officially licensed Amiga ROM and OS files are available both for download and on CD-ROM as part of Cloanto's Amiga Forever package, which includes Kickstart files up to v3.1 (the latest version).

You also need software to run on the emulator. Like a piece of hardware, an emulation by itself is not very useful.