One could talk for hours about the best way to go about starting GT but welcome aboard!
What you do from this point forward depends on where you want to go with the game.
For me the there are two big choices to make. First is, what driving aids you are going to use, if any? My suggestion would be to start where you want to finish. If you want to be a competitive driver in online racing or leagues you must start using only ABS=1 and nothing else for aids, as that is the only allowable driving aid in most series. I believe it also is the best way to learn car control and more importantly, while learning how to drive, you won't be learning bad habits that using aids can teach you. For example, SRF teaches you to push the car beyond it's limits because SRF is a magic, invisible hand, the sort of keeps you on the track and allows you to faster than the physical limits of the car. Drive the same way with SRF off, and you will likely spin out on every corner or at least threaten to. Traction control teaches you to just mash the throttle at every corner exit without consequence. Active steering helps steer the car. Etc. etc. etc.
Second big choice is tires. In open lobbies, most people throw RS tires on everything, from 275 hp Honda Civics to GT500 cars, LMP's and beyond. In my opinion, RS tires mask almost all the handling characteristics of every car and make them dull and sterile to drive. Lower powered cars go around the track practically on rails. If you do use RS tires most of the time, then try lower grip tires it'll be like driving on ice. If you mostly drive cars with their native tires, mostly CS and SH to RH, and then go up to RS tires it'll be a joke to you, there will be so much grip. I say avoid the bad habits RS tires teach you, and learn how to control cars on the types of rubber they were meant to have. It's much more rewarding, challenging, and ultimately you'll be a better driver.
Outside of that, drive, drive, drive. Practice technique, practice smooth above all. Wild = spectacular replay = slow. Smooth = less spectacular replay = lower lap times.