There's no need for monitor mode.
Just put the interface into managed mode
then connect to your network
iwconfig wlan0 mode managed
now you have 2 options:
arpspoof may be easier to use, since it's only meant for arp poisoning (ettercap has other functions, Google it if u want)
Usage: arpspoof [-i interface] [-t target] host
in the target you type the ip of the victim(the PC you want to sniff data from), and in the host you write the ip of your gateway. that will tell the victim that you are the gateway, so all the data will be redirected to you.
root@bt:~# arpspoof -i wlan0 -t 192.168.1.23 192.168.1.1
Now open a new shell, arpspoof again but now change the ip's like these:
target -> gateway
host -> victim
so that the gateway thinks we are the "victim"
Don't worry about the victim connectivity, arpspoof will redirect all the data it receives to the right ip address.
Now you can open wireshark and see all that data flowing like water
Hope it helps
And sorry for my english...