Quiero un billete is grammatically fine but lands like a demand. At a counter Spanish softens the request with quisiera or me gustaría, both of which mean I would like. Follow either one straight with a noun or an infinitive.
Examples
- Quisiera un billete de ida y vuelta a Madrid.
- I would like a return ticket to Madrid.
- Me gustaría viajar en primera clase.
- I would like to travel first class.
- ¿Podría decirme a qué hora sale?
- Could you tell me what time it leaves?
The mistake to avoid
Me gustaría is followed by an infinitive, not by que: me gustaría ir, not "me gustaría que voy".
Where this fits in
Travel & directions, Spanish A2: Get around in a Spanish-speaking city: take transport, ask the way, check into a hotel, and buy a ticket.