How does one draw a tile with diagonal lines that can seamlessly tiled with Photoshop?
Diagonal lines is definitely something that's a little more painful. But, I would recommend using similar techniques as to tiling photos. By using the offset feature you'll be easily able to tell if it tiles, if not adjust it.
Alternatively, there are some great free resources on the web that will create tiles diagonals for you with a huge level of control, my best recommendation is: http://www.stripegenerator.com/
Stripe generator I use as well. But if you want to teach yourself, open a new file in photoshop. Make it small, maybe 10x10 pixels. Zoom in until you can start to see the grid that represents pixels. Using the marquee tool, highlight exactly one pixel and use the paint bucket to fill in that pixel with a color. This will ensure crispness of the line. You can make a diagonal line with these pixels.. When you think you have something, go to the menu 'edit' and choose 'define pattern'. Name it something you remember. Then open a new file and add it as a layer style. For example... draw a rectangle, double click on the rectangle in the layers palette to show the layer styles, choose 'pattern overlay' then choose your stripe pattern you just created. this will then repeat it across your rectangle.
Here's a way to create hatched / diagonal pattern in Photoshop. You can also download a sample file. Check this out!
The pixel by pixel line tool and pen tool are not the easiest or best ways to do this at all. especially if you are going for a precise look that has transparency and other effects. Tutorials are the way to go.
Another thing to consider is learning how to create tile images from scratch, there is a separate tutorial for that.