To change a line type in Autocad, click on the line you want to change, then type in Change at the command line, then type Properties. Then type LT, then type Dashed, then type a number to give the scale. For example, I use 20 for fullsize drawings and anything between 1 and 5 for 5" gauge drawings, but it depends on what size you are actually drawing at, so as far as the number to set the size of the dashes, you will have to experiment on your own drawing. Specify a number then go to the drawing and see what has come up. It may look like a solid line but if you enlarge a portion of it, you should see if it is dashed or not, or if you need to reduce or enlarge the scale of the dashes. Hope that helps. If not, let me know and I'll try to describe it a different way. Autocad is a very complex program because it is one of the top professional CAD programs; certainly it is one of the earliest to be written. I started using it in 1989 with Acad 12.
Edit: I forgot to add...you don't need a new layer to change a line type from Continuous, to Dashed, or Center. (note the spelling of Center. It's an American program) unless you want it for, say, just dashed lines. I rarely bother with layers in Autocad, except to put all drawing lines on layer one (in Red) and layer 5 (in Black) for all dimensions. Extra layers just complicate things. Perhaps I should qualify this though. If producing a 3D assembly, layers to have each part on, are then almost an essential.