How do I say 'I'm happy'?

开心 (kāixīn) and 高兴 (gāoxìng) both mean “happy,” and in many contexts they are interchangeable. But the nuance:

• 开心 — inner emotional state, more personal. 我今天很开心 = I feel happy today (inner state, no particular cause needed).
• 高兴 — pleasure or satisfaction, often linked to an external event. 很高兴参加这个活动 = Pleased to attend this event (formal/host-like).

The intensifier choice matters: 很 (very) is the standard neutral intensifier; 好 is more colloquial and warmer; 太…了 is the exclamatory form. 好开心 said with genuine tone is the most natural everyday happy expression.

A cultural note: Chinese speakers often describe the reason for happiness rather than the happiness itself. 今天天气真好 (the weather is so nice today) is functionally equivalent to “I’m happy” in many conversations. The emotion is understood through the appreciation of the cause.