Blood is usually taken to rule out other general health problems which may be the cause of your seizures such as low blood sugar or other chemical changes.
The Dr may also want a blood test so there is a baseline of results ready for when he puts you on Epilepsy medications or you have a suspected infection.
Blood is taken using a needle and syringe, normally from the inside of your arm by the bend of your elbow. The test is quick and simple and all you will need is a plaster to stop the bleeding. The test is only slightly painful although those with sensitive veins from previous infusions will feel it more than others.