How to limit a column to 12 charactors (and delete any additional charactors)

I have a spreadsheet with 3000 rows by 1 column (so there is actually only one cell per row).

That cell in each row has 45 charactors in it (a different combination in each row).
I would like to keep the first 12 charactors of each, and delete anything after the 12th char in each row.

Does anyone know of a way that I can do this (a magical command or macro)? I thank you in advance.

Post sample data please

That worked perfect. Thank you so much!