You can create a new Dashboard layout from scratch if you have an abundance of time and energy on your hands. However, you will probably find it a whole lot easier to save one of the default Dashboards under a new name and then start the tweaking process. This method is not only a whole lot easier than starting from scratch it's also a whole lot safer; if you manage to mangle a Dashboard beyond use you can always start back at square one!
The first thing you need to do is decide exactly what data you want to appear in your new Dashboard. For example, you might be a Sales Manager and decide that you want to have a separate Dashboard for each member of your sales team; in that case the ACT! default Dashboard might be a good starting place. Or maybe you would like to see total sales and opportunities for your entire company and not just one individual; in that case the ACT! Opportunities Dashboard would be a good place to start.