The sample code below enables you to get the first template name associated to the given proposal.
/**
* The below code gets first template name.
*/
public String selectFirstTemplateNameForProposal(String proposalName)
{
String templateName;
Id proposalSOID = [SELECT Id FROM Apttus_Proposal__Proposal__c WHERE Name = :proposalName LIMIT 1].Id;
templateName = Apttus_Proposal.ProposalWebService.selectFirstTemplateNameForProposal(proposalSOID);
return templateName;
}