Please, thank you, and the basics of being polite.

• MesedezPlease
• Eskerrik askoThank you very much (most common 'thanks')
• Mila eskerThank you (literally 'a thousand thanks')
• Ez horregatikYou're welcome (reply to thanks)
• BarkatuSorry / excuse me (to apologize or get attention)
• Barkamena eskatzen dutI apologize (more formal)
• Ez dio axolaIt doesn't matter / no problem
• LasaiIt's fine / don't worry (literally 'calm')
• Egidazu mesede batDo me a favor (natural phrasing; literally 'do me a favor')
• On eginEnjoy your meal (said before/during eating, literally 'do good'; also 'On egin dizula')
• ZorionakCongratulations / happy birthday
• Ongi izanTake care (warm farewell, literally 'be well')