This is rather simple.
Next to the mixer, right click in an empty space and choose Add FX bus. Choose something like simple delay as the plugin. Now what you will want to do is choose a preset in the delay that does echo (long delay time) and then you will want to automate the send fx level (which is avalaible at the track level) You can automate the send level by right clicking on the desired track, choosing enveloppes, then choose your fx. Now you can raise the enveloppe at the word you want to apply the effect on. You can also globally control the send level by clicking on the pan control and choosing your fx instead.

Hope this helps,

Nat