It depends on the type of blood test you're having. The healthcare professional arranging your test will tell you if you need to do anything to prepare for it.
You can eat and drink as normal before some blood tests. But if you're having a "fasting blood test", you will be told not to eat or drink anything (other than water) beforehand. You may also be told not to smoke before your test.
Examples of blood tests that require you to fast include:
For more information about a wider range of blood tests, go to Lab Tests Online.