告 stacked an ox 牛 over a mouth 口: in one reading, the announcement made to the gods when an ox was sacrificed. Telling began as something solemn — you informed heaven before you informed people.
告诉 is the everyday 'to tell', but 告's formal edge survives: 广告 is an advertisement (wide-telling), 报告 a report, 告别 a farewell. To sue someone is simply 告 — informing the court.
An ox 牛 with a mouth 口 — solemnly TELLING the news.
广告 'wide-telling' is the word for advertising — so every billboard in China is etymologically an ox-sacrifice announcement gone commercial.