It's only worth monitoring your blood pressure at home if you stick to the correct procedure, like resting for at least five minutes before each reading and not taking readings in isolation but averaging out three consecutive readings.
There are two types of monitor:
an inflatable cuff that can be pumped up and the blood pressure is read of a dial
a digital electronic monitor that provides a digitally displayed blood pressure reading.
NB Only buy a monitor that is approved for use here in the UK.