To make sure we've got room in the forum for other stickies, I'm putting links to the Celtic 101 tutorials here.
Celtic Knots 101 - Triangles & Pretzels
Celtic Knots 101 - Running Borders
Celtic Knots 101 - Working In a Circle
Celtic Knots 101 - Spirals
If you're an absolute beginner, I highly recommend beginning with Triangles & Pretzels or Running Borders. And as always, if you need help with these, post or pm.
I am planning at least one more tutorial (Working with Irregular Spaces), so stay tuned!