There basicly are two ways echos can occur:
- Sound from your speakers/headphones being recorded by your microphone.
 - Your sound driver recording all sound that you hear for you.
 
Now,  'which one do I have' you might be asking yourself... You can find  out  quite easily: Just turn your speakers/headsets off (volume = 0%),  and  ask your mates if you are still echoing. If you are not, you got  (a); if  you are still echoing you have (b).
 
 Fixing (a):
 
 This type of echo is nearly impossible to fix completely, but you can  minimize it greatly if you follow a few general points:
- Switch your speakers off when TeamSpeak is on, use a headset to hear.
 - Use headphones that shield well, meaning they prevent as much sound as possible from being audible to the outside.
 - Try and keep your microphone as far away as possible from the headset (or more general, the sound-source).
 - Lower the volume of the sound-source.
 
Fixing (b):
 
 These echos are produced by your  sound-drivers, they should be  configurable by your driver too (its not a  bug, its a feature). From  windows users we have often heard you can  select from which devices to  record in the mixer (a mixer is the place  where you can slide all these  volume controls). Make sure the  microphone is the only entry that is  being recorded from. How exactly  this is called, and where exactly you  can find it depends on your  driver/OS combination, you just have to  search around. Good luck.