Treinamento começa às oito.

In Portuguese, especially in titles, schedules or bulletins, you can drop the article for brevity.
• You could fully say O treinamento começa às oito, but in a note or heading it’s common to omit O.

às is the contraction of the preposition a + the feminine plural article as.
It really means “at the hours,” since horas is implied.

In Portuguese, c before a, o or u is normally /k/. To show the /s/ sound before these vowels, we use ç.
So começar → começa keeps the /s/ sound in the third-person singular.

When telling time in Portuguese, you can omit horas.
• às oito = “at eight (o’clock)”
• às oito horas = same meaning, just more explicit

Yes. Both of these are correct and mean the same:
• O treinamento começa às oito. (Subject–Verb–Time)
• Começa o treinamento às oito. (Verb–Subject–Time; common in schedules/announcements)

Informally you’ll often hear treino.
• O treino começa às oito.
But treinamento is more formal or refers to a full training program.

Break it into syllables and stresses:
• Treinamento – trey-nah-MEN-too (stress on MEN)
• começa – ko-MEH-sah (stress on ME)
• às – [ash]
• oito – OY-too (stress on OY)
Put it together smoothly:
trey-nah-MEN-too ko-MEH-sah ash OY-too.