How much plywood would I need to make a track or a bank 180 degrees. I was thinking 4x8 or is that too much.
This is an unanswerable question as it stands. We don't know the room you have for a layout, what track configuration you're using, what your rolling stock is, what your minimum radius is for the most restrictive item in your rolling stock, what
gauge loading you'll need (you can look that up), and what kind of playing-with-trains you like or would like to learn to like.
However, to help you out, here are several big mistakes typical for newcomers to the sport...er hobby. It's really a sport at times because you have to have a thick skin and be a good sport to enjoy it, especially if you make one or more serious errors:
1. Not having the room for an adequate track plan based on your interests and rolling stock;
2. Not figuring a reach into a large layout surface to retrieve derailed items or to fix scenery;
3. Not leaving enough room to walk around the layout and to store stuff in the space you're allotted;
4. Not having enough safe distance from a broad curve that gets too close to the Drop of Death at the edge of a layout when things derail or just get knocked over by your elbow as you're trying to reach something further into the space of the layout (see how it all interrelates..?);
5. Laying curves too sharp for the lengths of engines and rolling stock. They become uncoupled or just drag each other out of the gauge...they derail. This is especially true when backing long passenger cars on tight curves. You'll forever be reaching over to rerail them because backing bunches up the couplers. If they have diaphragms, especially, you're asking for trouble on tight curvature.
6. Wanting a grade where the train passes over itself or a lower road and there is insufficient grade or insufficient overhead clearance for the train passing under, say, a bridge. If the grade exceeds about 3.3%, many locomotives will only be able to pull themselves and three or four cars, with some variance for heavier locomotives and lighter cars; and
7. Not making grade changes, from level to up, and from level to down, gradual enough that the coupler release pins don't snag or the couplers simply disengage. Not only that, but the gradual vertical curves into and out of grades eats up part of your total distance to that overhead you're trying to craft.