As you tell that you do not have proper documents for the project, probably what you could do is as a first step you can test the software from the customer point of view.
To do that what you need to do is
1. Identify your target customer
2. Think yourself as that customer.
3. Do the Usability study of that Product or project
4. Document all the Usability study for your future reference.
Hope this helps.
2. Software requirements is changing....
Even I have same problem in my concern. Because of which quality of the product becomes unstable.
To avoid this we need to do lot of exercise in designing phase which may avoid requirement change.
I usally run in to this all the time on my job. What you have to do is either two things.
1. Set-up meetings with key stakeholders and ask them a lot of questions about the project. In addition to that, go and have "Impromptu" meetings with individuals and get more information until you're comfortable enough to create Test Plans and Test Scripts. Also, let them know that your Test Plans and Test Scripts are based on the serveral meetings that were held during the life of the project (I would put this comment inside the Test Plans).
2. You can tell them that QA will not deliver any documentation until you have received the proper documentation from the key stakeholders of the project.