welcome to the boards Jedny
It all depends on who you are flying with, who you booked through as well.
If you go to the Virgin website (as I am making a BIG assumption its Virgin!)
http://www.virgin-atlantic.com/en/gb/index.jsp
On the righ hand side of the page, you can input the name of the lead booker, the Booking ref or E Ticket number (you should either have this on your E ticket or your travel company can give you the booking ref, not theirs, but Virgins)
This will take you to a page where you can view your seat preferences (or the preferences you TA has inputted for you) and change them. You can change for a red seat. Grey seats are either already booked or being held back until 24 hours before take off.
You can also input your advanced passenger information on these poages as well to save time at the airport.
Also, its worth joining the
Virgin Atlantic Flying Club (you can do this online at the web site) as this will get you miles from your flights which you can use for future discounts off flights, plus access to the FC call centres (which sometimes can be a blessing, other times.....)
If it is VS75 on 18th August
Economy is quite full, showing only 25 free seats and 12 of these are singles.
But don't forget that you will have seats assigned by your booking anyway