I would get the uniforms as soon as they are available.
For school supplies, my teachers usually give me a list of what they want during the first week of school, but I am sure many high school send out a list before school starts, so get the supplies as soon as you can- the beginning of August will have the most sales and school supplies will be in most stores.
A list of general school supplies:
One binder per class. 1 inch is usually the standard requirement, but 1 and a half inch is also common.
1 notebook per class. I usually buy 4 more than the number of classes I have(For example, if I have 8 classes, I get 12 notebooks), just in case I fill up a notebook.
Pens(blue, black, red)
Pencils(I prefer 0.5 mechanical pencils.) Papermate is my favorite company.
Extra lead and erasers for mechanical pencils and
If you need a new backpack, get one too.
Tab Dividers are EXTREMELY useful to stay organized.
A pencil case, if you don't already have one.
If your school usually wants you to get Expo markers, then get Expo markers, as well.
A lot of time, I like to write my notes in color, so I get Pilot G2 pens. They come in a variety of colors.
Hope this helped!Good Luck!