首 combines hair (巛 — three wavy strokes on top, suggesting locks of hair) with face (自 — originally the nose-pictograph from the 鼻 entry). Together: hair-on-face = head. The metaphor expands naturally to "leader / first / chief" — a head leads a body. Identical across 繁體 / 新字体 / 简体.

Note that CJK languages have a separate, more anatomical word for "head" — 頭 (Mandarin tóu, Japanese atama). 首 is reserved for the body part in formal/medical use, but dominates the metaphorical "leadership / first" sense in compounds.

Mandarin: shǒu, dipping 3rd tone. 首都 (shǒudū, capital — "head capital"), 首相 (shǒuxiàng, prime minister — "head minister"), 首先 (shǒuxiān, first of all), 首要 (shǒuyào, primary), 元首 (yuánshǒu, head of state). Also a measure word for poems: 一首诗 (yī shǒu shī, one poem) — quirky but rooted in the "head/first item" metaphor.

Japanese: on-reading シュ (shu) in 首相 (shushō, prime minister), 首都 (shuto, capital), 首脳 (shunō, leader). The kun-reading くび (kubi) is the everyday word for neck (not head!), with extended meaning "to fire/dismiss" — 首になる (kubi ni naru, "to be fired", literally "to become a neck"). Watch the meaning shift: 首 means head/leader in Sino-Japanese compounds but neck in native Japanese usage.

The derived character 道 (path/way — 首 + 辶) literally means "the head walking" — pointing the way forward.

Memory aid: hair (the three top strokes) above a face (the box below). The leader of the body.