How do I add volume changes in a specific track? I mean if I have a section where I want the drums and bass to be quieter and then after a certain number of bars increase in volume again, how do I do this?
Look up Read and Write Automation.
Hey 65 guitar, looks like RTFM is about as much as your going to get with questions like that, but then if you didn’t know automation exists then at least you got a pointer in the right direction. 
65 Guitar: I agree with mashedmitten, check out Automation in the Manual. You can draw in Volume changes for your drum and bass tracks. But here’s another way that can be Automated in your Mix with Read and Write Automation.
- Open your mixer
- hold down your Crtll key and
- Left click in an empty area of "THE COMMON PANEL and Hi lite that mixer strip.
- Now while still hoding down the Ctrl Key Hi Lite all the other drum and Bass channels
that you need in those channels COMMON PANEl’s. - Now Right Click any Hi Lited channel and select "LINK CHANNELS
When you change the level on one channel the other Hi lited Linked channels follow right along
and the other channels don’t
This is what I did to to my Rythm Tracks on 3 verses of a song to create Dynamic’s
just the other day
Hope this helps
Jack 
PS: READ THAT MANUAL and checkout cubase videos on http://www.youtube.com