When speaking to the King or the Queen, the proper form of address is your Majesty (or your Majesties). When speaking to the Prince or Princess, whether they are the rulers of a Principality or the heirs of a Kingdom, address them as your Highness.
It is customary to bow or curtsey to royalty when they pass by. Certainly bow or curtsey before approaching their throne at court. Depending on your kingdom, it may also be customary to pause and give a short bow even if you are merely passing in front of the thrones or royal pavilion. (The definition, in terms of distance, of "in front of" is left to the discretion of the reader.)