All businesses must be registered with the Accounting & Corporate Regulatory Authority (ACRA). This includes any individual, firm or corporation that carries out business for a foreign company.
A foreign company that wants to set up a branch for its business in Singapore will need to appoint two local agents to act on its behalf. These agents must be Singapore residents, that is, either citizens or permanent residents, or foreigners with employment or dependant passes. Information on registering a branch of a foreign company is available here.
Special licenses are needed for some businesses such as banking, insurance and stock broking. Special licenses are also required for the manufacture of goods such as cigars and firecrackers.
A step-by-step guide to registering a business or company in Singapore is provided at the business.gov.sg website.