Nassersalah26,you would do well to begin by looking at the sample database that Microsoft provides. See
http://msdn.microsoft.com/en-us/library/8b6y4c7s.aspx
Once you look at a few examples you will begin to see what you're up against.
There is no simple answer to your question. If you want to change a form you will want to start with the form designer. If you can find the list of form names, try right-click and choose Design. Or maybe you need to use the query editor if the problem is in a query behind the form. You might need to use the built-in VBA editor.
If you want to make one, small, isolated change you might get by with a little forum help. But if you want to do much at all you really should start with the sample database.
Jim