which pin should i connect to read a TTL signal from a external source ?
Any pin.
<1.5V will be read as Low.
3V will be read as High.
In between, will be indeterminate.
Use 2 to 13 as your first choice. But you can use A0..A5 too, and 0,1 reserved to Serial Monitor.
Do not exceed 5V.
If you have 12V for instance use this approach
or use two resistors as voltage divider.