High level Test cases and Low level test cases:

Write all the testcases under high level TC,which can be 
covered the main functionalities like 
creation,edition,deletion,etc....as per prescribed in the 
screen.

Wrtie all the testcases under low level TC,which can be 
covered the screen,like input fields are displayed as per 
the requirements,buttons are enabled or disabled,and 
testcase for low priority functionalities.

Example a screen contains two edit boxes login and password 
and a pust buttons OK and Reset and check box for the 
label "Remember my password".Now let us write high level TC 
and low level  test cases.

HIGH LEVEL TC
--------------
1.Verify that User is able to login with valid login and 
valid password.
2.Verify that User is not able to login with invalid login 
and valid password.
etc...
..
..
3.Verify that Reset button clears the filled screen.
4.Verify that a pop up message is displayed for blank login.
etc...
etc..

LOW LEVEL TC
--------------
1.Verify that after launching the URL of the application 
below fields are displayes in the screen.
1.Login Name 2.Password.3.OK BUTTON 4.RESET button etc..
5.check box,provided for the label "remember my pwd" is 
unchecked.
2.Verify that OK button should be disabled before selecting 
login and passwrod fields.
3.Verify that OK button should ne enabled after selecting 
login and password.
4.Verify that User is able to check the check box,provided 
for the label "remember my pwd".
etc..etc...

In this way,we can categorise all the testcases under HIGH 
LEVEL and LOW LEVEL.

1.check for availability of username,password text boxes and
  submt button
2.enter correct username and password and submit and check  
whether it is accessing your home page or not
3.password should not be display charcters and it should in 
dot format
4.if username and password dosent matches warning message 
should be displayed.
5.clear button must be there
6.forgot password button should be there
7.new user's option should be there

consider above 5,6 options according to the coustmer 
requirements................

testcaseid ,Description of testcase,steps to execute, input 
data,Expectedoutput are the column headings of testcase.
1. Verify the following text fields in the Loginscreen-
Username,Password.
2. Verify the following Buttons in Login screen-Login,Reset.
3. Verify the mandatory fields in Login screen.(without 
entering data in the text fields)
4. Verify the validation message shown when trying to login 
by leaving username field.
5. Verify the validation message shown when trying to login 
by leaving password field.
6. Verify the error message after logging when wrong 
credentials using.
7. Verify if user is able to login to Application when 
using correct credentials.
8. Verify the functionality of RESET button.
9. verify the look and feel of Login Screen.

Positive Testing:
1.enter the login window
2.verify user name,password text boxes enabled r not when 
we r ente into login window
3.verify submit button,cancel buttons are diaabled r not 
when we r enter int login window
4.enter the correct user name,password.
5.verify when we r enter password in the text box it 
displaying characters in dots r not
4.verify  submit butoon it is enabled r not when we r enter 
corrct user name and password.
5.when we r click the submit button its allowing next 
window r not.

Negatve Testing:
1.enter the user name, and leave password empty, the n 
click submit button.
2.verify when we click submit button with out enter text 
into the password it showing popup message r not
3.enter the username and password wrongly. an click submit 
button.
4.enter the password and username corrctly and click cancel 
button
5.verify when we r leave user name as a blank. and enter 
text into the password and click submit button