The requirements to get different credit cards may vary and some may even have a minimum income requirements. Of course, applicants with proper job or have income proof are more likely to be successful with their applications, but it does not mean that a self-employed or unemployed person will not able to apply for a credit card.
You may consider credit cards for students or of entry level. Most student credit cards, such as WeWa card and DBS COMPASS VISA Tertiary Student Card, only require applicants to submit identity proof, residential proof and a copy of student card. Credit cards with low income requirement, like Citibank Rewards VISA card and HSBC Visa Gold Credit Card, require the minimum annual salary of HK$60,000 only. Applicants just need to provide income proof of designated salary.