Hi JK,
Welcome to the world of 3D modeling. I'm glad you chose SE over the rest. I too fell in love with SE when i had my 1st encounter with it in my college days. I've been using SE since then at home and at work. However, modeling is only the beginning. You'll be facing great challenges ahead when you're ready to step out of CAD modeling and enter the realms of CAM & CAE. That's the interesting and challenging step.
Back to answer your questions, i suppose you're referring to create a distance dimension between 2 lines in the 2D sketch environment. 1st of all, the 1st line which you pick to place your dimension will be the line that moves to the specified distance from the other line.
However, in order to force a particular line to stay stationary, you can either use constraints like 'CONNECT' or 'HORIZONTAL' to fixed the line to another element or simply use the 'LOCK' constraint to fix the line at it's current position. Thereafter, the other line will move to the specified distance when you modify the attached dimension.
Hope this helps.
Andrew