I basically want to start a new line in design mode, but it comes up as a whole new paragraph when I click enter. Is there any code which I can place to start a new line directly underneath the previous one? Thanks.

I don't use DW but try holding down SHIFT and then click enter, this will give you a line break <BR />

Blue-Dreamer has this dead on.

In design mode the enter key will give you a new para, whereas, shift and enter will give you a new line, ie a br tag.