ok, tips.
first of all, do what you like. whatever grabs your attention, do it. worst thing that can happen is that you could do it easier, but later.
second. be prepared, or get prepared. basically, you will need two cars per category, or one car with two setups if you are tight with money, or really like a specific car.
one setup for the smaller, curvier tracks, and another for the faster ones.
trying to create a single car setup that wins everything, is a task you can always put yourself to, after you become experienced with the game.
also, if you get a D class car and pump it up to A class, i can bet that 9 out of 10 times, it wont be for its handling. so balance there too
you keep this in focus, and try to work yourself around the game, trying tuning solutions based on the above logic. and nope, automatic tuning is not optimal. it will help you if you never want to bother with this aspect though.
normally I could suggest you a couple of people's storefronts, to get suited up on your cars, but forza is not quite there yet on that subject, so your best bet is to locate good tunes through the dedicated forums. they will help you more than you know.
or, you can get an idea about "winning cars" by browsing the leaderboards, and just buy a car that you see in top100-top200 and you like, so that you know it definitely has the potential and experiment tuning yourself. the top100 leaderboard times, they all smoke even the most difficult AI for fun.
as you get more experienced, you will see that practically every car can be a career winner, and even be respectable on the leaderboards too.
a good tip to get you started on tuning: after you locate a -lets say- top 50 tune, buy a brand new OEM car to install it on.
then, when you go to tune menu, the game will say that in order to proceed, the other person's tune must be removed. so proceed, and while the car becomes stock again, all the parts the tuner used remain "bought". of course you dont get a way to have the tuning settings too, but having a top tune "reverse engineered" at least in parts, is a very good way to start get to grips with stuff. you can always reload the tune, and have it properly with the parts+tune settings again.
oh, and starting on lower classes kind of helps things remain simpler.
anything else you want, ask away