- you can practice (test) on any track at any time with any car, if you want to practice as close as possible to race/ series condition:
1) find these in series schedule
2) you can adjust weather and track conditions before practice or testing
3) if you join a practice in a series, official race, then it is already considered a participation (you could potentially back out (forfeit/ withdraw) before "practice/ quali and race"
4) series/ races depend on your license class (rookie, D, C, B....A and Pro)
5) and your participation in series and races all go hand in hand with your safety rating and iRating (
http://faq.iracing.com/category.php?id=9)
6) and of course you kind of decide which path to use -> Oval or Road racing (road also splits into close cockpit and open wheel vehicles)
in higher rated classes and iRacing Splits, you will quickly notice in races with setups, you will need good setups! (you can try this also if you are not much into setting up vehicles and want some help to start with:
http://iracingsetupsync.nickthissen.nl/)
hope this helps.
PS: iRacing has a very good manual to read everything in detail and also fantastic FAQ site.