The first fully finished seam I learned was a flat fell seam for pajamas.  This seam conceals the raw edges to prevent unravelling.  Sew a normal 5/8" seam allowance with WRONG sides together; trim one of the allowances to 1/8".  Open the garment and press the full seam allowance over the trimmed.  Turn the raw edge under on the full seam allowance about 1/4" and stitch on the fold through all layers.  This seam makes a great finish on a casual shirt too.