I don't change my tension - much. I used to when our kids were home and I was doing a lot of mending. Thicker fabric needs a higher tension, thinner fabric needs a lower tension.
As far as 'how much' tension - Place your fabric pieces under the presser foot like you are sewing a seam and give the fabric a slight tug. All you want is a little resistance - not a lot.
If you find your stitch length is not stitching to specs ('x' many stitches per inch) then play with pressure foot tension until you've got it right.