yes.first you need to select the test cases,which can be 
automated test cases.
The automation process focus on to the more number of test 
cases automated equal to less number of lines script.
1)Multiple sets of test cases will do the single operation
2)for performance
3)

First Catagerise to related each functionality. Then you 
decide which test cases are difficult to execute manually, 
which are regularly changing(code) functional areas then 
you mark those as automate, all remaining are manual 
testing.

For automation  we have to write testcases manually and 
execute them once.  then only we can automate.

automated test cases are those which are regression testing 
and retesting.  regression testing are done on theose whcih 
are resolve-fixed and resolved works for me and high 
priority test cases which is nothing but regression suite.  
while writing test cases we will categorise them.  so that 
we can easily identify those test cases which are to be 
automated and which are not.

if i am wrong please correct  me.

Actually what generally happens in tuomation Testing is 
that we first go through the different test cases and we 
only go for automation of those test cases that are 
difficult to handle manually in the long run and also saves 
our time in doing the testing effectively.