If you're a dude with dark hair and a beard already, then you've got it made! For the truly authentic look, you may want to throw some gel in your hair and slick it back.
If you're not naturally equipped with Billy Mays features, don't despair. You can buy fake beards and wigs online or at your local Party Supply store.
Any sort of long-sleeved blue button-up shirt will do. The important part is to print out the logo (Attached below) and tape it to the upper left side of your shirt. I'd recommend using double-sided scotch tape for a non-permanent attachment.
Billy always left the top two buttons undone with a crisp white undershirt underneath.
Simple khaki pants, preferably pleated, with a brown belt.
Perhaps you already use OxiClean and have the bottle laying around? If not, most pharmacies or department stores will have a variety of shapes and sizes to choose from.
Billy always wore a watch... You could probably get away without this detail, but if you have one - put it on!