Both of the games you picked are good, so you should be fine for awhile on that front. You might as well download RaceRoom Racing Experience (commonly shortened to R3E) since it's free-to-play and has a few cars and tracks you can mess around with for free. You can then go on and buy cars and tracks that you're interested in from there if you'd like.
As much as I enjoy iRacing, I would actually hold off on it for a short bit until you figure out if you like actually competing with people online, since that's entirely what iRacing does. If you do like competitive racing, I would definitely check out iRacing, since they do that really well IMO, and they usually have some promo codes for a few free months of service.
A word from experience with iRacing, though, is that if you get into it, maybe don't buy every piece of content that you think looks cool unless you've got money to burn. I made the mistake of buying a lot of the cars that I thought looked cool when I ended up not really racing in many of them, so I pretty much wasted my money. Also check out what cars and tracks the official series you are most interested in run and base your purchases off of that. I suppose most of this advice could also apply to R3E and any other F2P race sim there may be out there.
As far as actually getting good, I would recommend gradually work your way up from making sure you can keep the car on the track to actually racing competitively with real people. Start off practicing on a car and track combo that interests you. When I hit a wall and can't think of a way to go faster and improve my lines, I usually just search YouTube for the game, car, and track combo I'm driving and I usually find someone I can get ideas from.
The most important thing to remember is that you will only get better with practice. Unless you have real life racing experience or something, you will probably suck at first. That's okay, everybody does. When I first started, I couldn't even keep the car going straight on a straightaway. While I'm not going to say I'm amazing right now or anything, I can at least now keep the car on the road more often than not. When you finally nab your first race win, the amazing feeling you get will be worth all the prior frustrations.