Translation:I start to work at eight in the morning.
"I start work" is much more natural than "I start to work", which might be part of "I clock in at half past seven, but I start to work at eight...."
If it was 1 o'clock, it would be "à uma hora", but for all other hours (since the number is higher than one) it is "às duas horas" etc. in plural. (The contractions are "à" = "a a [uma hora]" meaning "at the first hour" etc.)