What do the blood pressure numbers mean?
As you probably know, the heart beats to pump blood through your body. When your heart muscle contracts to pump the blood out of the heart, the pressure in your arteries increases as the blood is pushed through. For your heart to refill with blood it needs to relax, and during this time the pressure in your arteries lowers.
When a doctor measures your blood pressure the result they record is a measure of how much pressure is in your blood vessels at both of these time points. The top number (the higher number) is when your heart pumps and is called your systolic blood pressure reading. The lower number is when your heart is relaxed and filling with blood and is called your diastolic blood pressure reading.
What is classified as high blood pressure?
A normal blood pressure reading should be roughly 120/80 (120 is the systolic number, 80 is the diastolic reading). Clinically high blood pressure is usually defined as being 140/90 and above. As high blood pressure can increase your risk of heart disease, diabetes and stroke it is important to keep your blood pressure as close to 120/80 as possible (a little lower is more than fine, but don't go too low as you can experience dizziness, lethargy etc.). If you have diabetes or any other medical condition, it is even more important for you to work towards keeping your blood pressure at 120/80 as your diabetes puts you at an already higher risk of heart attacks, strokes and other blood vessel conditions.
Which arm should I use for monitoring my blood pressure?
When I went through university I was taught to check peoples' blood pressure using their left arm. I'm not entirely sure of the reasoning behind this – I always just assumed that it has something to do with your left arm being closer to your heart than your right arm.
But a close look at the literature shows that the scientific community has known for a long time that blood pressure readings taken in your right and left arm give the same result – hence it shouldn't make a difference which arm you use to measure.[2,3]
Take a closer look at the literature and you'll suddenly notice that actually in fact we should be measuring both of our arms' blood pressures. Why? Because even though the blood pressure readings shouldn't be any different, sometimes they can be – and this can be an early sign of heart disease, kidney disease, diabetes and/or a heart defect. A difference of more than 20mmHg in the systolic reading, or 10mmHg in the diastolic readings should be followed up with your doctor. 
If the difference between your two arm measurements is less than 20mmHg in systolic readings or 10mmHg in diastolic readings, but there is still a consistently higher reading in one arm – use that arm to keep an eye on your blood pressure readings in future (with occasional measurements on both arms to make sure the difference between your arms has not suddenly increased).
How regularly should I check my blood pressure?
Unlike low blood pressure, people with high blood pressure do not usually have any symptoms. It is important then to regularly have your blood pressure checked to make sure you catch any increases early before they contribute to a heart attack or stroke.
If you have not already been diagnosed with high blood pressure, diabetes, a history of heart disease, stroke etc. then you can leave your blood pressure to be checked once a year during a regular check up with your doctor. If you do have any of the above conditions, or are pregnant, then it is important to have your blood pressure checked more regularly. Depending on what your most recent blood pressure test results have been, your doctor can tell you how often you should aim to check it in future.