The thing you want, order or buy goes in the accusative case. Feminine nouns swap -а/-я for -у/-ю; masculine non-living things and all neuter nouns look exactly like the dictionary form.
Examples
- Я хочу кофе и воду.
- I want a coffee and a water.
- Мы заказываем пиццу и салат.
- We are ordering a pizza and a salad.
- Дайте, пожалуйста, счёт.
- The bill, please.
- Я беру чай без сахара.
- I'll take tea without sugar.
The mistake to avoid
Leaving feminine nouns unchanged ("Я хочу вода") — it must be воду after a verb like хотеть or заказывать.
Where this fits in
Food & ordering, Russian A1: Order at a café or restaurant, ask for the bill, and talk about basic food and drink.