You have not indicated which version of Access you are using, but if you are using Access 2003 or less, then go to Tools/Startup. Select the form you want to have load when your applicaiton starts from the "Display Form/Page" combo box.
If you are using 2997, click on the big round button and then click on the Access Options button located at the lower right of the form. Then click on the Current Database option on the right. Locate the "Display Form combo box and select the form you want to load when your applicaiton starts up.