Repeat exactly what you have just said to us to your customer. Start a conversation, and take it from there.
In my experience, they will likely appreciate your openness and desire to make sure all parties interests are protected.
If they are large, they may have contracts/SLAs that you can use as a template, and if they are small they may agree to settle for some very simple terms. There are also many templates on the internet available for a small charge.
But as Robert says, you really shouldn't sign anything legally binding without some input from a qualified solicitor. Further, it can't be your run of the mill high street lawyer either. You really need somebody that deals with software/SLA's on a regular basis. Sadly in my experience these people are never cheap.
Allen