Nobody can hear you talking but you can hear others

There is several reasons why your microphone can not transmit your voice to the others:

Be sure that your microphone is plugged in the right place in your computer, most people it is the pink plug.

Check on your headset cable for a mute button, some headsets come with a mute button attached to the cable.

Check if your ventrilo is not on any mute settings, some mute settings are in the main window of your ventrilo and one other in the setup page.

Make sure that your microphone is selected as recording device

Click, Start, control Panel, Sound & audio devices and go in the tab Voice.

Click the "Volume..." button under the recording section, and adjust your microphone input volume and be sure that the Select box under Microphone is selected and nothing else.

Make sure that you have the right recording device selected in the setup of ventrilo.

Open you ventrilo setup window and select the input device corresponding to your computer sound card, if you do not know try all the listed choice one by one to see if it is your current problem.