EPS and PSD files are prefered, at the size that you want the design to be printed. A minimum of 150dpi is required and shading is not desired without additional artwork.
Artwork: Most jobs can't start, or even provide a quote, unless you know what type of artwork you want on the shirt and the rough dimensions that you want the design printed. 1-2 Solid colors, no shading, tends to work best because the artwork is the easiest to prepare. Also the standard maximum dimensions for a simple job measures between 10" width to 12" height. Lastly front and back printing on white or black shirts covers 80% of most jobs and if you want something different than that, then the dimensions and placement of the artwork should be discussed while acquiring the quote. Most artwork is emailed in an EPS, PSD or PDF file format at the size that you want the design printed. Send artwork to firstname.lastname@example.org