Plenty fluids definately
Get up from sitting/ lying to standing slowly.
If you do get really dizzy lie down elevate your feet, if your out sit down and put your head tucked between your legs.
Don't stand in the one spot if you are up walk around keep blood flowing.
^ yeah that.

I also found that if I looked up I would get dizzy, so avoid that too if you can.

Had low bp with both my pregnancies. Thankfully I never actually faired, just felt like I might.

Hope it gets under control.