Do not use br to break your lines. This is a very bad usage that will surely bites you back one day or another. Use Cascading Style Sheets (CSS) to define the presentation of your page. There a many good tutorials on TechRepublic and the web.
Dreamweaver provides tools to define CSS. Personaly, I prefer creating them by hand but the interface provided by DW will ease your way in.
An example: in the head of your page, you can add style definitions using:
This will reset all margins (top, right, bottom, left; the order is important) to 0;
If you wish to remove margin only on a specific group of paragraph, wrap this group in a div with an id.
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and then, the style definition would become:
#intro p {margin: 0 0 0 0;}
CSS is a very powerful tool. Go through some tutorials and grab a good book, and you'll be up and running very fast.