The following notes are from The Non-Designers Design Book by Robin Williams.
There are four basic design principals:
Proximity - items that are related should be grouped together
Alignment - every item on a page should have a visual connection with some other item
Repetition - some aspect of every page should be the same throughout a web site
Contrast - if two items are not the same then make them very different
These are the basics, master these and you do well.
Start | Proximity | Alignment | Repetition | Contrast | Assignment