As for when to check your blood pressure, the most important thing is to do it consistently the same time of the day (ask the doctor which time he prefers and also what time in relation to taking your medication). The following article has a lot of good information for someone just starting to monitor their blood pressure:
High blood pressure, also known as hypertension (See blood pressure chart below) is called the “silent killer” for a reason — there are no obvious symptoms but it can result in heart attack, stroke and even death. The good news is there’s a lot you can do to maintain healthy blood pressure or get back to one, often without the need for medications.
About This Image: Person receiving a blood pressure test. Medical research shows that as we age blood pressure rises slightly to accommodate an increased demand of oxygen and nutrients. It is completely natural for the first number (systolic) to be 100 plus our age. A recent study by a group of UCLA researchers came very close to corroborating Dr. Piette's guide for blood pressure of 100 plus your age for men, subtracting 10 for women, and this is after this rule had been in use for five or more decades. Are we now being taught that Dr. Piette's guide for blood pressure is wrong merely for drug company profit?

The guidelines also redefined the various categories of hypertension. It eliminated the category of prehypertension, which had been defined as systolic blood pressure of 120 to 139 mm Hg or diastolic pressure (the lower number in a reading) of 80 to 89 mm Hg. Instead, people with those readings are now categorized as having either elevated pressure (120 to 129 systolic and less than 80 diastolic) or Stage 1 hypertension (130 to 139 systolic or 80 to 89 diastolic).
Typically, more attention is given to systolic blood pressure (the first number) as a major risk factor for cardiovascular disease for people over 50. In most people, systolic blood pressure rises steadily with age due to the increasing stiffness of large arteries, long-term buildup of plaque and an increased incidence of cardiac and vascular disease.