As in the real world, your club must find a sponsor. A sponsorship can give you extra income each season.
To get a sponsorship you must send an application to the potential sponsors (Office > Sponsors). Before the start of the coming season the sponsors will respond to your application.
The sponsors have a limited budget for use on sponsorships. They try to obtain the best possible publicity per Credits the invest in sponsorships. The more vifa points your clubs have, the better publicity. The better publicity, the greater sponsor deal.
Club A, which has 1000 vifa points, sends a application of 1000 Credits to Sponsor X. Club B, which has 2000 vifa points, also sends an application. The sponsor chooses Club B's application, because it will get a better publicity per invested Credit.
You can send applications to as many sponsors as you want. A good idea is therefore to apply to one or more sponsors with a very low amount of Credits. In this way you can avoid entering next season without a sponsorship. After all, it is better to have a small sponsorship than none. When you have send a couple of "safe applications", you should send some more greedy ones in order to find your clubs optimal sponsorship requirements.
Remember that a sponsorship has duration of one season. This means you must apply for a new sponsorships every season.
You can only have one sponsorship at a time. The sponsors will set up sponsorships every Sunday evening. If you club get more than one sponsorship the most lucrative of these will be chosen for your club. The sponsorship is payed out immediately after the end of the season.