all really depends on what game you have since there are many different racing sims out there today.
Dirt Track Racing 2 (DTR2)-An oldie but still used by many for their dirt fix. This game is slowly getting phased out but you can usually find the hardcore online racers still on Gamespy from time to time. A lot of different cars and tracks to download for this game, which makes it appeal to all. Also, the $10 price tag appeals to many.
Nascar Racing 2003 Season (N2k3)-One of the better racing games ever made. However this game was discontinued and finding a copy less than $100 on eBay is very VERY rare. I used to have the hookup in getting it but unfortunately don't have that luxury anymore. If you can somehow get your hands on a copy of the game then finding a league to run in shouldn't be hard at all. Has many mods to it so you don't have to race the Nascar's specifically. There has even been a COT mod made for the game, making it more "up to date." Dale Jr even has his own site dedicated to his online racing league using N2k3.
rFactor-A VERY new game to the racing community. Mainly used for dirt late model racing (diRtFactor) so I'm totally out of the loop if there are any leagues out there to run. If curious, go over to
http://www.dirtfactor.net, sign up, and go into the forums and start asking question about the game and finding leagues to join. A few good mods out there for this game, which almost promises to be a hit here soon.
Also, to get a league started here through MSTRC you are almost going to need a dedicated server that you can host races on every week. Since a dedicated server costs money, you are going to need to find someone to pay for that server. If you want to go a cheaper route, you can find a racer with a very good internet connection (preferably cable connection) that wouldn't mind hosting the races themselves. The only downfall is if someone wants to practice with another racer online and neither has the high speed connection, their practice session is worthless since the connection speed they practice on won't be the same as what it would be on race day. Another alternative would be to run an offline league (like I wanted to earlier this year) so that everyone could have a league to run in and wouldn't have a dedicated time and place to meet every week, which some people hate to do or have an internet connection that is subpar.