Ok so first check if you installed subassembly composer.
This videos from mr Jeff Bartels can help you alot to get started with subassembly composer:
https://www.youtube.com/watch?v=eN5DtY9gs-w&list=PLjsw5HrA4MXn0qTwXCASOBlnYeG2jN8KI
The red dot on your right screen when you open subassembly composer is representing your centerline with elevation from your profile view. As you build your geometry it will appear on that window.
Subassembly is controlled with points. To add point to a subassembly you just pick it from left menu and drop it in work flow. Arrow will appear. Arrow control directions how you define your ditch.
In the right bottom of a screen you will see parameters, to add a new one you just click on create parameter.
For your ditch you will need 3 parameters. Bottom width of a ditch, slope of ditch and height of a ditch (note: you can't use special letters and space in name of parameter). When you create parameter you have a drop list to choose from: Double(1.02, 2.56 etc), Integer (1,2,3,4), Grade ( in percent) , Slope (1:2, 1:4), String (cat, dog) and so on...
Geometry is defined by points from geometry menu on a left side of your screen. You will see that you can choose a type of point ( Slope and x, slope and y, x and y, angle and x or y ) and from what point to set that distance and slope.
So first go and create parameter Width and set as in a picture. Then you drop a point and set a type (x,y) and put it from origin at X=0, y=0. Then you drop a second point and set type of point again to x,y. Now under x value you write Width/2 and it will creat a point from P1 that is half distance of your width. Then you create third point same as P2 and under x value you set -Width/2. Now you have 3 points representing the bottom of your ditch. Next step is to connect point P2 and P3 with line. Again you take a line from geometry menu and put it in a work flow.
When it comes to coding, you do following:
Under the code of P1 you can write "Center line" you need ". Codes in points will build feature line in civil 3d.
For a line code set "Datum" or "Top", so you can create surface from it.
Hope i did help a bit, if you have more questions just ask
