If you have problems clearing your ears on descent, then you are having difficulty inflating your middle ear via the Eustachian tube which connects it to the back of the nose. Provided you have normal hearing on the surface, then your Eustachian tubes must be working properly as the air in your middle ear is continually being absorbed and replaced by fresh air coming down the tubes.
There are several things you can do to help make it easier to clear your ears.
Firstly, practice inflating your ears several times a day for three to four days before you intend diving but don't do this if you have a cold as pushing infected mucus into the middle ear is not a good idea. Regular use tends to open up the Eustachian tubes. Next, when in the water, start inflation from the surface downwards and for the first few metres with each breath. Never wait until there is a pressure on your ears and, if you can't clear or if there is pain, go back up until the pain and pressure has gone and try again.
Never, whatever happens, go on down in the face of increasing pain or pressure on the ear drum. If you ignore this advice, if you are lucky, you will get seeping of serous fluid into the middle ear, leaving you mildly deaf for a few days. If you are not so fortunate, a burst ear drum may result, or if the worst happens, a ruptured round window or intralabyrinthine membrane with hopefully short term tinnitus, vertigo and almost certainly lifelong high note deafness with inability to hear such things as telephones, doorbells and violins. If you are unfortunate enough to suffer tinnitus, vertigo, and deafness after a dive, and you think that it might be diving related, seek the advice of a diving medical physician immediately - time here is of the essence.
If you are diving from a boat down an anchor line or down a well secured shot line, then it often helps to pull yourself down feet first at least for the first ten metres. That way the column of air in your chest and throat exerts positive pressure to help open the Eustachian tubes.
Medication, in the form of a tablet of Sudafed taken half to three quarters of a hour before entering the water, may help and is safe to take when diving to a depth shallower than about 30 metres. Sudafed is an "over the counter" drug. However, this drug is absorbed into the body and occasionally produces side effects. It is wise therefore to try it out a day or two before diving. Ephedrine 1% nasal drops used before diving may also be helpful and should be available over the counter.
Lastly, if all else fails, there is surgery. Many people with poor Eustachian function have abnormal bony spurs extending into the back of the nasal cavity and removal of these often cures the problem. Others have a crooked nasal septum which can be straightened surgically.