There's a couple of ways to do this. One is to just take each item one at a time and multiply by the sales tax rate and see what it would be on that item. Since that very easily might not add back up to the total tax due to rounding, you might have to adjust an item up or down to make it add back up to the right total.
If you have lots of items but only a couple of categories, just add up all the prices of the stuff in each category and use the total to figure the tax.
Your question actually asks "what percent of the sales tax," although that's a harder method for most people. But you can do that. You want the sales tax to be in the same proportion as the prices of the items. Add up all your taxable items to get the total. Divide each item (or category of item) by that total, which will give you the proportion that item is of the total. (Like 2.00 item divided by 10.00 total is .20 proportion, or 20%.) That's the "percent of the sales tax" that applies to that item or category. So you multiply .20 by the total tax, and that tax belongs with that item/category.