You initially start with only access to the Rookie servers, but if you race well enough in your first few Rookie races your ranking/rep will increase so that you will then have access to the ranked races (that are only eligible if you have the required REP).
Your starting rank is 1500 and starting REP is 70. Each race you run then recalculates your ranking/REP from this intiial starting point.
It seems the ranked system has a few bugs at the moment since the latest update went out in December. While the races are available to race, the results are not being processed, so no-one's ranking is being updated. They are working on getting this resolved.
They were hoping to process all the results racers submitted since the patch, but this is not possible, so some will have raced a lot of ranked races for nought (but on the flipside, bad races will also not be counted).
While the rankings are not currently updating, they posted in their Discord channel that the servers had been restarted and were working on getting the rankings to update after each race (still a WIP) but have stated that since yesterday's server restart, once they have run through the re-processing of results, any races done since yesterday morning will be populated in your ranked race history.
If you go here -
https://game.raceroom.com/multiplayer-rating/ - and scroll down the page a little, you will see all the ranked servers that are available.
Obviously you will only be able to join the ones your ranking allows. Also, you can only join the servers while they are in Practice session. Once it moves to Qualifying, the lobbies are locked down to stop further racers entering. Quitting out at any time will impact on your ranking.
If you have not run any ranked races so far, it will only be the Rookie servers that will be available, as they have no REP limits.