People can hear me but i can't hear them

You do not say what your audio setup is - if you have a laptop you are using the built in speakers or a desk top with add-on speakers. Also you do not say what your OS is

Here is Vista x laptop x Yahoo Messenger
TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon (checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard
If no sound is detectected, click on Speaker icon to highlight, click 'Properties' ramp up sliders - test again

SET DEFAULT & TEST MIC
Click "Recording" tab.
Set required mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click OK
Click "Levels" tab and set slider to 100% (adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays
Restart computer

CONFIGURE TO MESSENGER
Open messenger - on top toolbar. Left click ( > ) Messenger > Preferences > Connection - Select 'no proxies' or 'connect direct to the internet' depending on the version of messenger you have
make sure your internet connection and country are correct in the lower two boxes > Apply > OK

Click ( > ) Messenger > Preferences > Calling and Audio
Set mic and speakers to show Default equipment
Run call up assistant to test audio equipment > Apply > OK

If you do not have Vista - you will have to access your audio equipment via the Control Panel