FS-UAE is an Amiga emulator that runs on Windows, Mac OS X and Linux. It supports emulation of Amiga 500, 500+, 600, 1000, 1200, 4000, CD32 and CDTV, including support for floppy images (ADF and IPF file formats), CD ROM images (ISO and BIN/CUE formats) and mounting local folders on your computer as hard drives. This last feature is very handy, as it makes copying files to your virtual Amiga machine very easy.
FS-UAE provides wealth of configuration options. Most of them are obvious and understandable, but you may want to have a look at the documentation page.
To create an actual Amiga virtual machine, you will need a copy of the actual Kickstart ROMs and floppy images of Workbench. FS-UA website contains information on how to purchase these, or use Google to find other sources.
I suggest having a look at The Green Amiga Alien Guide for a great tutorial on setting up very useable (including Internet access) Amiga 1200 virtual machine. It was written for WinUAE emulator, but I found it to be also of good use for FS-UAE.