Rule of thumb is to sit with the heel of your riding shoe on the pedal at its lowest position and your foot parallel with the ground. If you can move your knee back and forwards without moving your backside off the pedal your saddle is roughly at the right height.

You can then adjust your saddle fore and aft so that your knee is in the right position above the pedal. There are two schools of thought on this: one says that a vertical line from the front of your kneecap should pass thro' the centre of the pedal spindle; t'other says that the bony bit on the outside of the top of your lower leg should be vertically above a line drawn through the centre of the pedal spindle. If you see what I mean

If you set up the height as the above and either of the fore and aft positions you will be reasonably well set up for most purposes other than long rides. If you get into the latter, it would be useful to take your bike to a bike shop offering a accurately measured setting up.