Some blood tests need to be done on fasting samples. You will be told at the time you book your appointment if you need to fast for your blood test.
If you are to fast, you should have nothing to eat or drink (except plain water) from 10pm the night before your blood test appointment.
To stress this point: water is allowed; food, tea, coffee, milk, lemonade… in fact, anything but water is not allowed and may mean you will have to book another blood test.
If the prospect of having blood taken fills you with fear, tell us, because there are ways to make the procedure more comfortable.