How long until I will notice a change in my blood pressure?

The BP machines at the pharmacies are uncalibrated nearly 99.9% of the time. I don't think I've ever seen one that is working properly. The other problem is that you've been walking around the store, which is going to raise your BP.

You need to take a reading AT REST, with your arm at chest level, legs uncrossed, in a seated position. At rest doesn't mean you sit down and take it, depending on the person you may have to wait 5-10 minutes.

Low sodium intake is good, but more important than that is adequate potassium, calcium, and magnesium in the diet. What foods are high in these minerals? Dark leafy green vegetables - spinach, kale, collards, beet/turnip greens, chards, etc. Pile on the vegetables (and some fruit). Corn is not a vegetable.

Stay away from sodium as much as you can. Sweets and chocolate and caffeine also raise your blood pressure. If you use tobacco product stop. They all raise it higher than it should be.