I have a custom tab . What I want to do is : when I click on that tab, it should open a VF page that I made. How can I do that ?
If you want the contents of the tab to be the VisualForce page, you need to make a VisualForce tab, not a Custom tab. In setup, VF tabs are at the bottom of the Tabs page where Custom tabs are at the top. I'm pretty sure you can't turn an existing tab from Custom to VF.
According to the Creating Visualforce Tabs documentation.
User Permissions Needed To create Visualforce Tabs: “Customize Application”
You can build Visualforce tabs so that users can access Visualforce pages from within Salesforce.
To create a Visualforce tab:
- From Setup, click Create | Tabs.
- Click New in the Visualforce Tabs related list.
- Select the Visualforce page to display in the custom tab. If you have not already created the Visualforce page, click Create a new page now.
...and then there are several other fields to fill in on the UI.