In ladder logic, and other PLC programming 'languages' as well (e.g. STL), the order is basically from top to bottom and from left to right.
But, keep in mind that most (all?) PLC's update the 'real' I/O's at the start/end of a programming block. This means that if on line 1 you change an output to '1', on the next line to '0' and the next line to '1', that the -actual- output only changes one time only to the last state ('1') and does not do 1-0-1. This is because during the programming block an *image* of the inputs/outputs is used, which you will manipulate. At the end of the programming block this image is than used to update the actual outputs, and the image is updated according to the real inputs.