I would suggest looking at websites like simracing.gp and Low Fuel Motorsport. These are typically your best bet for getting into races quickly. On Low Fuel Motorsport you can find races that run every hour or so, you just need to have a good safety rating to start racing. Simracing.gp has different events, leagues, and tournaments going on that you can join, but races are less frequent over there compared to Low Fuel Motorsport. Both are good and let you get into actual online races, so take a look at both, and you should be able to start racing.