To frame photos and watercolors yourself, you'll need frames, mats, backing boards and glass. Other materials and equipment: hinging tapes for pictures and for mats, self-healing mat, rulers and right angles, x-Acto knife, possibly a beveled knife, a burnishing bone, hard graphite pencils and erasers.
Frames: framing and art supply stores sell frames in wide range of styles and materials. Nielsen aluminum frames come in several molding widths and profiles and several colors -- silver, brushed silver, anodized black, white, and brass or gold, too. Nielsen frames are sold by chop pairs with assembly hardware included, the price depends on molding and chop length. The length readily available ranges from 5" for $3 to 40" for $15 a pair, with increment of 1/16", but the standard industry allowance is 1/8". To get a frame, you have to order two pairs, obviously. The dimensions you give are dimensions of your matted art, not outside dimensions of the finished piece. Nielsen frames are plain, simple, precise, easy to assemble and well made. I like how they frame photographs and ink drawings.
Mats: matting board comes in several thicknesses, or plies. 2-ply is most common, 4- or 6-ply allows deeper matte opening. It looks more dramatic, it's sturdier, and it's twice as expensive as 2-ply. Quality matting boards come in white, off-white, and black with standard (white) or black core. Other colors are available in unpredictable quality. Note that white and off-white mats of different brands can be very different; use same brand of matting board for series. Matting board should be acid-free and archival, cotton rag (the best) or alpha-cellulose. Art supply stores sell precut mats with standard overlap of 1/8", but I cut my mats myself. It gives more flexibility and certain immediate satisfaction of doing things myself. And it's cheaper, obviously.
Mat-cutting knives: vary greatly in price and options. I use very basic beveled knife to make rectangular matte openings with slanted sides, Logan 2000 push style. It costs about $26. It allows different depth, but only one bevel angle. The ruler I use is a steel straight edge 3" long with cork lining. I use burnishing bone to smooth and hide small over-cuts in the corners.
Self-healing mat, also called cutting mat, rotary-cutter mat (marketed for tailors and quilters) or self-sealing mat. Comes in variety of sizes. Sizes from 8"x12" to 24"36 available in art and craft supply stores. 4'X6' mats exsist, too. The mat has firm, dull surface with contrasting alignment grid on one side. Most common brands are Scotch, x-Acto (grey), Olfa (green), and Omnimat (usually green). In my experience, grey Scotch mats are thicker than green Olfa. Self-healing means the surface sustains repeated cutting with sharp blade. Note that a dull blade can visibly damage the mat, beside ruining whatever you were cutting. Store the mat flat, don't bend it or roll it, don't let it freeze. Wash it with wet soapy sponge or paper towel. I keep 24"x36" mat "wrong" side up as my working surface all the time. Cutting mats cost from $9 for a discount pink 12"x18" Omnimat to $60 for 24"x36" x-Acto.
Backing board: usually foam board, corrugated cardboard or corrugated plastic. Foam board is lightweight and easy to manage. Sells precut in standard sizes or in large sheets. Has to be archival. Cut it the same size as the matted art if you cut yourself. Nielson frames come with additional springs which fit under the sides of the frame and press the picture against the front of the frame.
Hinging tapes: Filmoplast or Lineco paper mending tape is fine archival paper adhesive tape. It looks like tracing paper and completely invisible after burnishing onto the back of you picture. It is strong enough for a photo. Fabric-based Lineco hinging tape is used to hinge the front of the mat against the back. It can be removed, if needed. Paper-mending tape is not removable, but it blends into original paper.
Glass: better acrylic. It weighs less than glass, it's more impact-resistant, it doesn't break in sharp dangerous pieces. It costs twice more. Just three weeks ago we had an earthquake, and an 22"x28" picture I framed myself landed next to my son. Were it glazed with glass, it would've cost me much more then just replacing the glazing. Framing acrylic is available with UV-blocking and glare-reducing coatings. If you are going to cut acrylic sheets yourself, you will need a plastic cutting knife. A plastic cutter by Fletcher costs $8.
See also: book repair, Speedball, India ink.