FIFA 08's online ranking system is rather confusing to some people with the main question being “Why aren't I ranked?” The answer to this is that there are only 10,000 ranking spots, so you have to be better than at least the ten-thousandth ranked player in order to have a rank.
Ranking is based on the number of online points you have accumulated. The main thing that determines points is whether you won the match or not. Other factors come into play when determining how many points you will receive such as: goals allowed, goals scored, possession percentage, and many others. To be honest, nobody but EA really know what exactly determines the amount of points you will receive after games, but I'm pretty confident winning games is the primary factor.
To check your rank, go to Main Menu - My FIFA 08 - FIFA 08 Profiles - My Xbox Live Career. If your rank shows up as simply “-”, then you haven't broken into the top ten-thousand mark, so you'll need to win a few more games.