There's an art to matting and framing a picture, just as there is to taking a picture. (Speaking mostly to color pictures here) To really enhance a picture, you need to find complementary mats and frames. Some people will mat a picture to match a room, and kill the picture in the process. And not all framing stores here have skilled employees at selecting mats to match the picture, so your eye will need to come into play.
Take your picture down to a shop which has mat samples (usually 6 x 6 corners with the bevel pre-cut), and just lay them down until you find something that works. Many color pictures can use 2 different color mats without a problem. I have a friend who's really good at this, and recommends looking at the colors out of focus. He does this by circling his fingers to about a pencil sized hole, and squinting through it. Blurs the image and lets you concentrate on the colors.
When I matted pix in the past, I usually took 11 x 14 pictures up to a 16 x 20 frame, so I had generous mat showing. I usually put and extra amount "under" the picture (1/2" or so) so it didn't look like it was sliding down out of the frame.
I prefer simple metal frames to ornate ones for my outdoor pictures. For portrait work a wooden frame was often used.
I buy my frame stuff from pictureframes.com, not sure if that's an option price wise to ship to you. I cut my own mats using a Logan Compact mat cutter, which saves $$ compared to doing it with a frame shop.
I am not selling my pics professionally, so doing it yourself may not be the best option for your cost structure.