how to draw steadier lines in photoshop?

The pen tool is probably what you're looking for. It can make perfect lines (even curved), but can be tricky to learn how to use effectively at the start.

It's too complicated to explain it here, but if you do a search for pen tool tutorials there are a lot out on the net.

edit: If you've got a Wacom tablet there should be a program with it that you can adjust the sensitivity settings in. I tend to turn the pen tip settings close to the "hard" side of the graph when it comes to Photoshop, because I too find it too sensitive on default.

If you use the paint brush and hold down shift, click then click some where else while still holding down shift it will draw a line between the tow points. It doesn't matter on what angle, it can be diagonal if you want. I don't know why when you did it it only created vertical and horizontal lines.

No questions asked, the best site for Photoshop tutorials is: http://www.photoeditinglessons.com/

All tutorials are in video format, which makes them easier for you to follow along with the instructor. The videos are interactive and very easy to follow, this will help you learn piano three times faster. Good Luck!

When you get better hand control, you'll be able to draw steadier lines. Or you can use vector graphics where it allows you to draw smooth curves.