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 ?
I managed to solve the situation.
I went to my custom object's tab, on the " Buttons, Links and Actions". I went to the tab line and clicked "Edit". And from there, I was able to override the standard page.
Note : The page used when overriding a tab shouldn't use a standard controller.
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.
...and then there are several other fields to fill in on the UI.