Generally, as a business providing marketing services, you will need to have a Service Agreement that is tailored to your business' requirements.
The agreement should set out the scope of your marketing services, IP ownership, termination rights, payment terms and limitation of liability. These matters will differ across businesses, so it's wise to get legal help to ensure you're receiving adequate legal protection.
It will also help ensure that both you and your clients are on the same page about the specific services you're providing and how everything will work.