How do I install qemu?

Just replace qemu with qemu-system-i386 or qemu-system-x86_64 as appropriate (whether you want a 32-bit or 64-bit system, and which ISO you're using).

You can also use aqemu, which is a graphical (GUI) front-end to qemu.

On Ubuntu 12.04 (Precise Pangolin) I have installed the qemu-system package with:

sudo apt-get install qemu-system

Inside the package it includes:

/usr/bin/qemu-system-cris
/usr/bin/qemu-system-mips64
/usr/bin/qemu-system-mipsel
/usr/bin/qemu-system-sparc
/usr/bin/qemu-system-sparc64
/usr/bin/qemu-system-sh4
/usr/bin/qemu-system-ppc64
/usr/bin/qemu-system-ppc
/usr/bin/qemu-system-m68k
/usr/bin/qemu-system-arm
/usr/bin/qemu-system-ppcemb
/usr/bin/qemu-system-sh4eb
/usr/bin/qemu-system-microblaze
/usr/bin/qemu-system-mips64el
/usr/bin/qemu-system-mips

I have executed an ISO image in an i386 machine with:

qemu-system-i386 -net user -cdrom my_iso.iso

In /usr/bin, there is not qemu, but you can use qemu-system-x86_64,qemu-system-arm , etc.

You can write:

qemu-system-x86_64 -localtime -net user -net nic -m 256 -cdrom minix.iso -hda minix.img -boot d

But if you need to use qemu, create a link to qemu-system-x86_64 in ~/bin/qemu.