Normally, hearing echoes is caused by other users having an open microphone combined with loudspeakers.
It is very important and necessary to use either a headset or headphones. Without them, people that speak to you will hear an echo of their own speech (since your speakers will be playing what they say and your microphone sending it back to them). This will delay communication and distract whom you are speaking to.
The quickest remedy to this issue is to simply plug in a pair of headphones with a built-in microphone. Check your computer's sound settings to be sure that the input and output are correct.