The basic choices are metal or wood. Most often the choice between those two are obvious from the outset. If it isn't here are the guidelines. Metal frames are modern looking. They go with photos, children's art work and needlework, humorous pieces and stylish stuff.
Wood frames are for Victorian pieces, outdoor scenes, religious pieces and serous stuff.
The metal frames should match or compliment the mats which compliment the piece. Wood frames should be as light or as dark as the mats are and be fairly close to the type of wood in the room they will hang in. Frames also come in many widths. The fancier frames are best used on complex, intricate work not photos and modern work. If you have problems with mat selection and this is all taking just a little too long, walk away from your work and look at something else for a few minutes.