Yeah, this was a weird game for me at first (I know nothing about cars).
Here's what I did (I don't know if you'll want to do it this way or not)
Basically when I started... I didn't even realize there was online, or at least.. I wasn't sure how it worked. I played A Spec a little, and then I got bored of the game, hated on it.. and almost sold it. I bought a Mustang, and was happy for a little, but meh...
Once I went back on like a month later, I just went online with a friend... did Daytona a bunch, using the cars in the garage that weren't mine... (Jeff Gordon mostly), and racked up a lot of money and EXP that way. I basically only play on Daytona...once I got enough for a Ford GT, I bought it.. and tuned it all the way, made it awesome, then I repeated that process using MY GT, and now I also have a Mercedes McClaren, (sp)...
I've done a couple of the A Spec cups, since they do technically give more money (Expert and up), but I find online way more fun playing with friends... or even just random people with mics.
Now I've got 2 nice cars, and am working for a third. That's how I got to a decent level, and you could too, lol. I really don't care for single player, but it gives more money - like I said - once you're a high enough level.
I reccomend using the nascar preset cars online for a bit, till you can get a car with at least 500HP from the Dealerships, then tune the crap out of it. By then, you should be able to do the Expert cups with ease, and then of course, the Extreme ones. : )
Also, B Spec becomes insanely easy at this point, just keep setting it up while you're doing something else like watching TV, and it's really easy money / XP.
tl;dr Online w/ best default first for money, then get a car with over 500HP at least.. and tune it, then do w.e you want.
That is my story.

Edit: You get cars for doing licenses and A Spec/ B Spec... here's a list:
http://gametipcenter.com/gran-turismo-5-unlockable-cars