There are several components to an SLA. It is important that the language is familiar to your environment. Use a straw man to introduce the concept to your organization. This will help the rest of the project team understand the complexity and importance. The following is an example of the categories that should be included in your SLA. However, remember that it must be clear, well written, and easy to read and not very long.
a) Purpose of the SLA
b) Mission of IT (This is not just between the Support Center and customers)
c) Customers covered under the SLA
e) Owner of the SLA document and communications path
f) Services covered. This is only a high level statement.
2) Service Goal.
a) Overall goal. Example: Resolve 100% of all severity level 1 and 2 cases in the timeframe specified. This is the first place that you are making a statement about what is important. You cannot be a success treating all cases equally. Level 1 and 2 are high severity level cases that directly impact the productivity of a number of customers/locations of the company. You must make sure these get done on time. Getting the others done on time is a bonus.
b) Specific goals. List the severity levels and the time the case should be responded to and the time it should be resolved.
Here is a great White Paper on Implementing Service Level Agreements. See https://www.givainc.com/white-papers/index.htm