It is your job to send applications to the sponsors, in which you declare your club's sponsorship aspirations. If your requests are too high compared to other applications they will be rejected.
The trick is to send applications with high aspirations, however, not so high that they will be rejected.
When you send an sponsorship application you must fill out "base amount", "bonus amount", and "bonus requirement". Base amount is the number of Credits you will receive no matter what (if the sponsorship application is accepted that is). The bonus amount is the amount of Credits you will get as a bonus if you reach the goal specified as bonus requirement (for example to end in the top five of your league). The sponsors will include the probability of you fulfilling your bonus requirement, when assessing your application.
You applications can be changed at any time throughout the season.