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
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.