The basic seam allowance is 1cm, however some closed seams need different allowances, these need to be marked on your pattern. Thick fabrics or those that fray may need a 1.5cm seam allowance.
Seam allowances on a pattern piece are indicated by a notch which is marked by a small snip into the fabric to help with matching pattern pieces. Notches should not be snipped as deep as the seam allowance, with more delicate or open weave fabrics, the notches can be marked with a fabric pencil or chalk.
Remember to reverse stitch at the start & end of every seam to stop it unravelling. The majority of seams in garments are sewn with the right sides of the fabric together.
Pictured in the images below is an open seam using 1cm seam allowance.
With right sides of the fabric together follow your seam allowance & stitch.
wrong side 
right side
Press the seam allowance open
For garments that are lined there is no need to finish the raw edge of the seam allowance.
For unlined garments the raw edges can be overlocked.
wrong side 
right side
Garment Construction 