you need to make a basic decision first: do i try to get as a high a licence as possible, quickly (because e.g. i want to run the big gt cars) or do i rather take my time, get to know the environment, pick up a bit of experience here and some there. both approaches in my eyes are reasonable and will lead to satisfying results.
for the first, the fast trip do a couple of mazda cup races but by all means start from the pit (ask in this thread if you don't know how to) and stay a second clear of everything that moves. in other words: run races that produce 0sr. this should quickly get you into the rookie 4.0 category. next step up would be the cadillac in the world series, however this is a very tough environment, lots of chaps who do not take a considerate approach. again, start from pits, stay out of trouble and after 5 or so clean events you should be up for promotion to c-class. at that point i would buy the ruf since you get four rides for the price of one and two of the tracks you need to compete in the ruf cup. again: stay out of trouble, just finish cleanly. all the time you will be picking up experience on how iRacing plays and feels. forget your racing instinct, that'll come later.
for the second: do your mazda events to get the d-licence, then buy the skippy (skip barber formula ford) and some tracks (there is a good thread on which tracks to buy in the skippy thread), read all the info in the skippy thread and spend two seasons in the skip. this should teach you all you need to know. after that, pick your target and by that time you will know how to get there on your own.