You've found a great resource here. You can bathe a Yorkie at three weeks, and you can bathe them when you feel they need it, but of course not too often as it gives them dry skin. Just make sure you use a shampoo that is gentle, like baby shampoo or special dog-formulated shampoo. They have good brushes at the pet store, we got ours at Petco, just ask the people there what kind of brush is good, I don't exactly know how to explain what kind of brush we have.
Congrats on your first Yorkie! There's many members here that may help with any questions you may have. Welcome to YorkieTalk!