Does anyone know how to find out who using my network other than me in ubuntu? Because I just want to make my wireless network being use only for the people I approve it?
Without knowing anything about your hardware or Ubuntu installation you really do not need any special software to find this out, you should be able to log in to the wireless router using any web browser and see a list of the attached devices.
This Lifehacker article has it pretty much covered.
You can restrict access to any router by MAC address filtering though a better method is to use a strong password and WPA encryption.