You need to use automation.
First you want to add an FX track. If you don't know how to do that you need to right click on the mixer or and select Add Track -> FX Channel Track.
When the dialog comes up asking you to choose a plugin, pick the desired reverb or delay plugin. You need to set that plugin to 100% "mix" or "wet".
Then, once you've done that you have to link the FX channel to a send on the channel in question. This is done by selecting the "Sends" tab on the left hand side inspector in the project window and clicking on the first slot. You should be able to select the FX channel from there.
Now, there are two ways that you can write automation:
1. In realtime using the "W" and "R" buttons on the relevant track with either the mouse or a midi controller.
2. Manually, by writing it in with the mouse.
To achieve this in realtime, all you need to do is select the "W" (red) and "R" (green) buttons on the relevant track and navigate to the sections in the song where you need the reverb to come in. Then, press play and use the mouse to send the appropriate amount of level to the FX channel. This is a performance thing so make sure to send some level and bring it down when the part is over while listening back to make sure it's not too much or too little. Once you are done, un-click the "W" button and leave the "R" button active so that Cubase will follow the automation you just programmed.
To write it in with the mouse is a bit more complicated but basically at the bottom left of each track is a little button that appears when you point over it and this will add an automation track. Click it once to reveal one automation track. On the automation track you can select which parameter you want it to control. Usually there is a list of 20 or so common ones but if it's not there you have to click "more" and find the relevant parameter. Note this has to be done AFTER you have enabled the send in the inspector. Once you have selected the right parameter, which in this case will be "Send 1 Level", you need to press the "R" (green) button on the relevant track so that you can manually write automation for Cubase to follow. You can then use the pencil tool to write in the appropriate amount of level at the appropriate place.
Phew.
I didn't expect automation to be so difficult to explain but I guess it is. If I were you I would use method 1 as it's easier and forces you to use your ears in realtime to assess the results.
Hope that helps.
Cheers
