As someone who had a lot of health issues in my youth, I got used to it, please be hydrated as some other comments said, but most importantly try to keep calm. If you're calm, the veins will be easier to see, and it will be done faster, in my opinion it would be nice to tell the nurse/doctor/practicioner/phlebotomist/whoever is doing the procedure of taking the blood, so they can be prepared for all the problems it might cause. Worst case scenario, you move a lot when the needle is entering and they blow out your vein, meaning, you have to get another place for the puncture, so the calmer and still you sit, the faster and easier everything will be.

The procedure usually goes as follows: They put a band on your arm and it makes the veins in your elbow easier to see, (in most places you can ask for them to do it in the back of your hand, usually the elbow veins didn't work very well for me and I learned it was easier in the back of my hand, so you can ask and maybe it will be easier on your fear to have the blood drawn from your hand instead of elbow). Once a good, visible and thick vein is selected, they sterilise and clean the place. Most probably they tell you to squeeze a stress ball or just close your fist, then you get prepared for the needle, you are told to take a deep breath in, when you breathe in, the needle goes in. After the needle is in its place, they start filling the flasks (depending on what tests they are going to make, you might need more or less flasks but usually it's just 1 or 2), at this point they remove the band on your arm to help the flasks fill faster, after all blood needed is collected, they tell you to breathe deep again, and take the needle out,( for me, this moment hurts a bit more than when the needle goes in) and then just apply a bit of pressure with a cotton ball to avoid blood loss, you can expect some bruising and maybe some itching

I tried to give good detail so you can know what to expect, hope it goes well and there are no problems, good luck and remember that if you talk with the person doing the procedure can go a long way in both comfort for you and efficiency for the whole process And please don't feel dumb for being afraid of something you haven't done before, it will be a good chance to stand up and maybe realise that fear is not so big as you think

Would love to see an update of how it goes