For the first time to visit Japan, you may be in trouble with buying groceries at supermarkets. This following table may help you how to explain what you want to buy in Japanese.
Where is the onions? = Tamanegi wa doko deska? = 玉ねぎはどこですか？
I want plastic bags = hukuro kudasai. = 袋ください。
*several kinds of vinegars
米酢 = Rice vinegar
リンゴ酢 = Apple vinegar
<Hint for Supermarkets>
There are many kinds of supermarkets in Tokyo. Seiyu(西友), Maruetsu(マルエツ) and AEON(イオン) are one of the famous supermarket brands. Moreover, there's good grocery shop that sells coffee beans and imported foods. It is Kaldi. You may have seen at south exit of Nishi-kasai station or other places in Tokyo. There're good coffee beans, coconut milk, cheese, chocolate and even some kinds of spices.