How does the ribosome know which reading frame to start on?

Answer 1

There is a certain sequence called a Start Codon, usually ATG or AUG, that the ribosome binds to. After that, it simply starts plugging away. If there is a deletion or insertion mutation, the reading frame can be disrupted.

Answer 2

E P A

tRNA enters the ribosome, and starts the process of inputting amino acids. it starts on the A, and exits the ribosome from the E.. assuming you're talking about reading the mRNA molecule..