The perfect stitch is balanced, no top thread visible on the bottom and no bobbin thread visible on the top.  All tension adjustments are made with the top thread tension dial.  The higher the number, the tighter the top thread will be and vice-versa. 
There is no tension dial for the bobbin and there is a tiny screw on the bobbin case for tension adjustments.  If you mess with that one, have fun getting it back to where it should be.  Such adjustments are best left to an experienced sewing machine  technician. 

If you want to do bobbin work, it is best to have a second bobbin case dedicated to that purpose.   BOBBIN WORK is a technique where the bobbin holds a heavier decorative thread that is hand-wound onto the bobbin and the stitching is done on the wrong side of the fabric, laying the decorative thread on the right side.  A lot of upper-end machines come with a second bobbin case with looser tension to accommodate the heavier thread.

EVERY RULE HAS AN EXCEPTION.  If you are basting or doing any line of stitching that you will be removing, loosen your tension so the top thread goes down toward the underside.  This will make it easier to remove the stitching by simply pulling on the bobbin thread.  Don't forget to return to a balanced stitch for your general sewing.

OR TWO:  If you are doing machine embroidery, either freehand or machine driven, and your bobbin thread is peeking through the top, loosen the tension to send the bobbin thread more to the underside where it belongs. 
Don't overdo it.

A word about loading your bobbin into the bobbin case. It is surprising how many sewing machines come in for repair because the bobbin area is a tangled mess.  "Gee, it was working fine, but now all I get is a birds nest in the bobbin case." And more often than not, the answer is simple---the bobbin is in backwards.

Look at your bobbin case--that's the part where you insert the bobbin. If your machine has a front loading bobbin, remove the case and look at it from the side, open area on top. There will be a slot in the case that angles either from top right to bottom left or top left to bottom right. When you put in the bobbin, the thread should be coming off the bobbin from a direction that will require a 180 degree change in direction when pulled into the slot.  In other words, if your slot runs top right to bottom left, your bobbin should be inserted with the thread coming off in a counter-clockwise direction so that you are approaching the slot from the left to the right and must change direction to the left after going into the slot.  Hold your thumb on the top of the bobbin to prevent it from spinning and continue pressure on the thread until you hear it click into the tension plate.  Conversely, if your slot is from top left to bottom right, reverse the motions with the thread coming off the bobbin clockwise.  It is this 180 degree change of direction along with the tension plate that puts tension on the bobbin thread. It is amazing how many times the incoming "repair" doesn't get past the check-in table when the problem is in the bobbin area.  I love those easy fixes.