Which do I apply first, concealer or foundation?

Apply foundation and then concealer. By applying the foundation first, you see exactly where you still need more coverage. And as you apply the concealer, you will see if more or less is required.

How do I select the proper foundation shade for my skin?

The best place to test match your skin tone is on the chin and lower jaw area.  To test a color, apply a small amount of foundation and feather out the blend. If the shade isn’t quite right select the next shade up or down in tone. Expect that the foundation chosen in mid-summer may be too dark in mid-winter. The objective of foundation is to smooth and blend skin tone and color differences to a natural healthy look of natural skin. If it looks like a mask, (or feels that way) than too much has been applied. If you see no difference, you have applied too little. Remember foundation should help you look simply beautiful.