buying a shoe bigger than your regular shoe size will not help you walk in them, it will only make it more difficult. make sure the shoe fits you perfectly and it is a quite supported shoe. now when you are just learning I suggest you start with a smaller heel, so the three inch in your case I suppose. start by practicing at home, not in public. make sure you are walking on a good sturdy floor (not slippery, carpeted, no rugs, no boards or nails sticking up, and floor is cleared and there is enough space). make sure you have good posture (you can check in the mirror just to be sure). keep your shoulders back an hands at your side. place one foot in front of the other but do not cross them over one another. do not have two separate paths for your feet, have them follow. sway your arms slightly as you walk along. keep practicing, eventually you will get used to it an it will feel more natural. when this happens you can go in public or work on the next inch higher.
Noo don't buy bigger. Buy which ones feel comfortable and fit. Getting ones that are bigger will hurt your knees/legs and make it so much harder to walk in them. You just need to practice wearing them. Wear them around the house to get used to it and break the heels in.
you have to remember to glide,not bend your knees like you're walking on steps. try to imagine you're walking on a straight line,putting your foot in front of the other foot with every step. you need good posture,straighten your back,push out your chest and butt,and just glide.
Get the right size shoe, with a 3 inch heel in either wedge or stacked heel (they are more stable) and wear them for two weeks one day off and one day on. Once your feet get used to them wear 4 inch heels and once you feel comfortable in them try stilettos (if you want).