How to know what all capabilities i need for my application.

This process is called "PlatSecDiagnostics" and is performed on the emulator.
Steps are
1. Edit EPOCROOT/epoc32/data/epoc.ini File
2. Check that you have this line
PlatSecDiagnostics ON.
3. Run your application with steps your application have to follow.
note: emulator will not give any error.
4. EPOCWIND.OUT from windows temp folder.
5. If you inspect EPOCWIND.OUT file you will come to know which capabilities are required.
For Example

    *PlatSec* ERROR - Capability check failed - A Message (function number=0x000000cf) from Thread  helloworld[10008ace]0001::HelloWorld, sent to Server !CntLockServer, was checked by Thread CNTSRV.EXE[10003a73]0001::!CntLockServer and was found to be missing the capabilities: WriteUserData . Additional diagnostic message: Checked by CPolicyServer::RunL