Hello Friend,
I will clear all of your doubts and Guide you to proceed
testing:
It is very common in small size companies development will 
not follow process.I mean process means they should define 
& follow some software development model(SDLC).
As part of this SDLC they should prepare Requirement
(BRS,SRS,FRS) documents, Design documents,Prototype Model 
GUI screen 
shorts,Flow diagrams,Database E-R diagrams..etc.
Now i will come to ur problem.

Ask development(Project Manager/Dev Team Lead)team for 
above requirement documents.
If u got the requirement documents u can easily develop
testcases.

If in the case even the requirements are not clarly defined
then sit with development team ask them to explain the
functionality of the application right from starting to 
ending,Including direct navigations and Alternative 
navigations.

Understand the Main application development purpose and 
concentrate to break the application.

Notedwon the Application different functionalities,
Different ways to use that functionalities..etc.

First start writing positive testcases
Then write Field level validations,then write GUI testcases.

Use Equivalent Class partition and Boundary value analysis 
to seperate valed in put and invalid inputs,then based on 
inputs prepare the positive and negative testacases.

When writing negative testcases verify  error alert 
messages for different error/Blank input values.
For example:
If you are writing testcase for login page:

Log in inputs can be devided into to classes(Equivalent 
Class Partition technique)

-----------------------------------------
Valid Class      |   Invalid Class       |
-----------------------------------------
Valid UserID     |  Valid UserID         |
Valid Passwrod   |  Invalid Passwrod     |
                 |                       |
                 |  Invalid UserID       |
                 |  Valid Passwrod       |
                 |                       |
                 |  Invalid UserID       |
                 |  Invalid Passwrod     | 
                 |                       |
                 |  Blank UserID         |
                 |  Valid Passwrod       |
                 |                       |
                 |  Valid UserID         |
                 |  Blank Passwrod       |
                 |                       |
                 |  Blank UserID         |
                 |  Blank Passwrod       | 
 -----------------------------------------
We have two classes now then first write positive testcase 
for Valid Class inputs:
the write negative testcases for invalid input values.

Ok Byeee

Rajendra Prasad Reddy
rajendra_penumalli@yahoo.com
Hyderabad 

Still if u have any dobut feel free to Mail me.
i will reply you soon.
Byeee

Hi Buddy,
  really its good scenario for beginner..