1. select the curve, right click your mouse and you will see some options pop up, here you want to select either vertices, vertexes or curve points (depending on the type of curve). You will then see small purple points, you can move these about to edit the shape of the curve.

2. You can either merge it to another nearby vertice and reposition it, or sometimes they can be deleted and it will make a 3 sided poly.

3. You could create a poly plane for a rectangle, or select one of the edges that make up the shape and select 'fill' from either the 'edit mesh' or 'mesh' drop down menus under the 'polygon' option set ( I think this would work, haven't got Maya open at moment but I'm pretty sure it's something along these lines).

I hope this helps you get started with Maya, I'm just learning Max after using Maya for about a year so I may need your help soon lol!!

Chris