I would suggest doing as much offline as you can... it will get you used to the cars the handling and short cuts/routes ... online is great but some people are wary of newbies (mainly as they are trying to complete all the online challenges) and they will kick you out fast .
When driving offline if you get a new car then change into it and try to find the burning route for it, winning this gives you a "hot" version of it. Its good to try and get this one asap as it gives you a good understanding of the various cars you can drive, and its needed when you get your ELITE license later on.
When you do races pick a car that your happy with .. the computer racers are matched to your car, so picking the fastest car you have doesn't make it that much easier as the computer racers will be similar spec too. I used one car for each of the different types of races as I used to like the handling of it
Lastly add all the guys in these threads to your friends list and join in when we are online ... I promise not to kick you out, like I did to Monty once