I just turned my raspberry pi on connecting only monitor and mouse to it. I have a power supply of 5V 1500mA. I am still on the screen where you start installing from NOOBS
A USB mouse not working is very weird, since they are standardized -- unless the mouse is very weird, e.g., gaming mice.
To try and see what is wrong you'll have to get past the NOOBS configuration or whatever to a full fledged operating system. I believe Raspbian comes already installed so that is probably the easiest. If the mouse still does not work,
sudo apt-get install usbutils lsusb
This should show you a few things most of which are internal (eg., the pi's USB hub). See if it looks different with the mouse plugged in vs. unplugged (a mouse will usually be pretty clearly labelled).
Next, unplug the mouse and
tail -n 20 /var/log/syslog. Then plug the mouse in and do exactly the same thing. This shows you the last 20 lines of messages sent to the system logger; it includes stuff from
dmesg, which is from the kernel. It may indicate some kind of USB error.