This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means "stand" so what is the verb version of that? It's to stand.

You have to remember the た portion of this word to learn the reading of it. Think of it this way:

You have to stand up in order to wave goodbye to someone. When you stand up, you also say ta-ta (た)!