A list of 5 things you should totally avoid while designing (and coding) a website:

1) Default colors

I know it’s not always easy to pick the right color for text and background, but seriously, some colors are just ugly in any situation. I guess part of the problem are the design software themselves. When you open the color picker, you always get a choice of default colors. These are the colors you should avoid. Even Microsoft took care of the problem. In Office 2007, they got rid of the usual colors found in highlighters and replaced them with some more professional tones:

Office 2007 Colors

If you still have no idea which colors to pick, you can always try Kuler from Adobe: http://kuler.adobe.com/

2) You could put some more padding here…

This one is quite frequent for beginners. You just coded a box and inserted some text. But the text goes right on the border…

More padding please

If you never looked at CSS, now is the time. Use at least 8px and you’re good to go.

3) You gotta love that high-contrast monitor…

Always use low contrast gradients when you’re going to put text above it. If the contrast is too high, no matter what the color is, it’s going to be unreadable.

Gradients Contrast

Trick: Use the first color you picked, open the advanced color picker and choose the color 5-12 steps above/below.

4) 1 box + 1 box + 1 box + … = Too many boxes

Unless you plan on making a CMS style design, you don’t need to put a box around everything.
Keep it simple, one box for your sidebar and another for your content. Use bars or bigger text to separate content.

5) Left we go

It was fine 10 years ago but now with monitors being larger and the resolutions reaching the 1920×1080, it just doesn’t make sense anymore to put a website aligned to the left.
Eyes are focused at the center of the screen, so either center align your website or make it liquid style so it fills the entire screen.