Service-Oriented Computing allows new applications to be developed by using and/or combining services offered by different providers. In several cases a service needs sensitive information from the clients in order to execute. The existence of a trust relationship between the client and the provider determines which restrictions the service has concerning the use of this information by the service. In this paper we present a metamodel for services computing that incorporates trust relationships. The metamodel provides the means for specifying services and service requests including trust requirements and constraints. The proposed metamodel is encompassed in a framework for goal-based service discovery and composition.