Sample Code:

//Get all record types on Account object
List<Schema.RecordTypeInfo> recordTypeInfoList = Account.SObjectType.getDescribe().getRecordTypeInfos();
String defaultRecordTypeId = '';
for(RecordTypeInfo info: recordTypeInfoList) {
    //Check default record type
    if(info.isDefaultRecordTypeMapping()){
        defaultRecordTypeId = info.getRecordTypeId();
        break;
    }
}
System.debug('DefaultRecordTypeId-' + defaultRecordTypeId);