Basic tips for the pen tool?

The pen tool seems so frustrating to use. It seems really touchy, but it's probably just because I don't how to use it properly. Here are my main issues. I'm gonna use this photo album to kind of explain my problem.

1. How do I switch from curved to straight lines with the pen tool? (Fig. 1).

2. How do I get rid of jagged points like this? (Fig. 2). All I'm trying to do there is create a simple banner.

3. Let's say I make a square using the pen tool (Fig. 3). Let's say point B was my last point. If I were to add another point, it would create a line from there. But how would I select point C and draw a line from there? If I use the pen tool and click on one of the points, it just removes that point. Using the direct selection tool doesn't help any either. Also, if I've deselected that path, how do I select it again and keep working on it? If I select the square and try to use the pen tool, it starts a new path.