Create Trigger Crash Course?

You can use the below code :

trigger kickOffWorkflow on CustomObject__c(before update){
	for(CustomObject__c newRecord : trigger.new){
		CustomObject__c oldRecord = trigger.oldMap.get(newRecord);
		if(oldRecord.CustomField__c != newRecord.CustomField__c){
			//Write your logic here 
		}
	}
}
//Please replace following varibales :
// 1. CustomObject__c with API name of your custom object on which you want to execute trigger
// 2. CustomField__c with API name of your field on change of which you want to perform some action

Please adjust your requirements in above trigger accordingly and let me know if you need more help on this.