It's easy, just read qemu-doc.html.
But if you need just quick answer:
qemu-system-i386 -cdrom /path/to/iso -hda /path/to/file.img -net user -daemonize
More advanced example (which I always use):
kvm -name vm -nodefaults -monitor vc -cpu host,-svm -m 1024 -drive file=file.img,media=disk,cache=unsafe -drive file=/path/to/iso,media=cdrom,readonly -net nic,model=rtl8139,macaddr=02:04:06:88:fa:bc -net vde,sock=/tmp/kvmnet.vde -vga vmware -daemonize
instructs KVM to provide host CPU definition into VM, add 1G memory, put a disk first then cdrom second, add RTL8139 NIC with specified mac address and attach it to VDE switch, then add vmware virtual GPU to have more video modes available and daemonize it.