On Line Check In

We Have pre allocated seats on our Virgin Atlantic Holiday flight, On previous Virgin Atlantic flights we checked in normally at the Airports.
I now understand to ensure getting the seats I must check in on line at least 13 hours before departure.
This only involves return flight as we are checking in Night Before at Gatwick.
As someone who has never done on line check in and is a complete novice on anything to do with computers ect,
Could someone help me please
1. Is my information correct
2 Exactly What details do I need to enter.
3 Am I able to use a 3rd parties equipment.
4 What is a sequence number I read about.
Sorry for such a long winded FIRST posting but at my age I need all the help I can get.
THANK YOU.
I now understand to ensure getting the seats I must check in on line at least 13 hours before departure.
This only involves return flight as we are checking in Night Before at Gatwick.
As someone who has never done on line check in and is a complete novice on anything to do with computers ect,
Could someone help me please
1. Is my information correct
2 Exactly What details do I need to enter.
3 Am I able to use a 3rd parties equipment.
4 What is a sequence number I read about.
Sorry for such a long winded FIRST posting but at my age I need all the help I can get.
THANK YOU.