In design view, open the properties dialog. Click the button on the Toolbox matching the control you wish to alter the defaults for (for instance TextBox), then the properties dialog should display "Default Text Box" - alter the properties and...
Also, if you've created lot a form or report you wish to be the default, or template for later forms or reports, in Tools | Options - Forms/Reports tab, set the form/report as template.
Roy-Vidar