Purchase an artist portfolio. Online art companies such as Dick Blick, Jerry's Artarama and Cheap Joe's sell artist portfolios in different sizes and designs. Right now, just worry about getting a simple 11" × 8½" presentation case such as the Prat Start 1 Presentation Case.
Get copies of your artwork. The best way to do this is by taking quality digital photos of your artwork. Take your art outside at around 4 o'clock for the best lighting and shoot each piece several times. You want to produce a photo that has the exact same colors as your original painting.
Crop and adjust the photo's contrast with photo editing software, such as Photoshop, Corel MediaOne or Photosuite, if needed.
Load the pictures onto a memory stick and take it to a print shop such as Kinkos or Office Depot.
Have them print high-quality 11" × 8½" prints of each piece of art.
Put the prints into the portfolio and you are done.