IF you will menstion SeeAllData = true its means your test class can access your org data. But is false then you need to create all test in your class only.

As per salesforce best pratice always try to create all data in your test class only. You should  not depend on your org data as once you will try to deploy same test class any other sandbox in that case your test class may fail or you will get less code coverage.

IsTest(SeeAllData=true) Annotation
use the isTest(SeeAllData=true) annotation to grant test classes and individual test methods access to all data in the organization,

If a test class is defined with the isTest(SeeAllData=true) annotation, this annotation applies to all its test methods whether the test methods are defined with the @isTest annotation or the testmethod keyword
The isTest(SeeAllData=true) annotation is used to open up data access when applied at the class or method leve

Please check below post for test class. I hope that will help you
1) http://amitsalesforce.blogspot.in/search/label/Test%20Class
2) Code Coverage Report in excel Format | Test Class Result in XLS (http://amitsalesforce.blogspot.in/2015/09/code-coverage-report-in-excel-format.html)
3) Test classes with @isTest (http://amitsalesforce.blogspot.in/2015/09/test-classes-with-istest.html)
4) Best Practice for Test classes | Sample Test class (http://amitsalesforce.blogspot.in/2015/06/best-practice-for-test-classes-sample.html)
5) @testSetup ( Set Up Test Data for an Entire Test Class ) (http://amitsalesforce.blogspot.in/2015/05/testsetup-set-up-test-data-for-entire.html)
6) StartTest and stopTest Method (http://amitsalesforce.blogspot.in/2015/02/starttest-and-stoptest-method.html)


Please follow below salesforce Best Practice for Test Classes :-

1. Test class must start with @isTest annotation if class class version is more than 25
2. Test environment support @testVisible , @testSetUp as well
3. Unit test is to test particular piece of code working properly or not .
4. Unit test method takes no argument ,commit no data to database ,send no email ,flagged with testMethod keyword .
5. To deploy to production at-least 75% code coverage is required
6. System.debug statement are not counted as a part of apex code limit.
7. Test method and test classes are not counted as a part of code limit
9. We should not focus on the  percentage of code coverage ,we should make sure that every use case should covered including positive, negative,bulk and single record .
Single Action -To verify that the the single record produces the correct an expected result .
Bulk action -Any apex record trigger ,class or extension must be invoked for 1-200 records .
Positive behavior : Test every expected behavior occurs through every expected permutation , i,e user filled out every correctly data and not go past the limit .
Negative Testcase :-Not to add future date , Not to specify negative amount.
Restricted User :-Test whether a user with restricted access used in your code .10. Test class should be annotated with @isTest .
11 . @isTest annotation with test method  is equivalent to testMethod keyword .
12. Test method should static and no void return type .
13. Test class and method default access is private ,no matter to add access specifier .
14. classes with @isTest annotation can't be a interface or enum .
15. Test method code can't be invoked by non test request .
16. Stating with salesforce API 28.0 test method can not reside inside non test classes .
17. @Testvisible annotation to make visible private methods inside test classes.
18. Test method can not be used to test web-service call out . Please use call out mock .
19. You can't  send email from test method.
20.User, profile, organization, AsyncApexjob, Corntrigger, RecordType, ApexClass, ApexComponent ,ApexPage we can access without (seeAllData=true) .
21. SeeAllData=true will not work for API 23 version eailer .
22. Accessing static resource test records in test class e,g List<Account> accList=Test.loadData(Account,SobjectType,'ResourceName').
23. Create TestFactory class with @isTest annotation to exclude from organization code size limit .
24. @testSetup to create test records once in a method  and use in every test method in the test class .
25. We can run unit test by using Salesforce Standard UI,Force.com IDE ,Console ,API.
26. Maximum number of test classes run per 24 hour of period is  not grater of 500 or 10 multiplication of test classes of your organization.
27. As apex runs in system mode so the permission and record sharing are not taken into account . So we need to use system.runAs to enforce record sharing .
28. System.runAs will not enforce user permission or field level permission .
29. Every test to runAs count against the total number of DML issued in the process .


Please let us know if this post will help you