Depends on exactly what you are trying to do.
http://msdn.microsoft.com/en-us/library/aa370557.aspx
You may need to rebuild the installer package with whatever it is you want in it. Now, if the installer includes a UI for choosing what components to install, you should be able to set the defaults. Note that orca probably won't work well on packages created with an SDK published after support for the orca tool ended. Otherwise, look at all the entries which refer to the components and registry entries to be created, and delete the ones you don't want. You may have to play the trial-and-error game until you have removed everything you don't want without breaking the installer.
Again, you may just want to re-author the package. General reference and places to find the SDKs: http://msdn.microsoft.com/en-us/library/ee663259.aspx
edit: As far as using orca itself, look through the interface, and see the help file. Help tells you about everything you need to know. (I just installed a copy and looked.) Bear in mind that you open the msi as an archive and see what it looks like from that angle, if it helps you at all.