depends on what level you are up, personally I bought my first lot of content so I could take part in the SFR season (D License) and it give me a few track I could do the global challenge (in the free caddy) and Grand Touring Cup (in the solstice) but to be honest buying the Skip Barber would be the best thing to do as they have races every hour.
Past D class I went on to buy the RUF, this allows you to take part in the RUF Challenge, GT3 and world sports cars.
This time I signed up to a local racing league and bought the tracks I needed for their season.
(remember, complete 8 out of the 12 official races in d class and about and you will get 4 bucks back to the max of 10 bucks per season)
Hope that helps