Я хочу is blunt at a counter. Russian softens it with бы after the past form: я хотел бы, я хотела бы. For needing something, the person goes into the dative and the thing stays the subject: мне нужен билет.
Examples
- Я хотел бы билет туда и обратно до Москвы.
- I would like a return ticket to Moscow.
- Мне нужен билет в первый класс.
- I need a first class ticket.
- Не могли бы вы сказать, когда отходит поезд?
- Could you tell me when the train leaves?
The mistake to avoid
Нужен agrees with the thing needed: нужен билет, нужна книга, нужно место.
Where this fits in
Travel & directions, Russian A2: Get around in a Russian-speaking city: take transport, ask the way, check into a hotel, and buy a ticket.