Following are two basic settings (I believe that there are other language specific, C++ for instance, settings as well).
For Java:
Window->Preferences->Java->Code Style->Formatter->
Click on ‘New’ to create a new profile and select the profile that you want to copy
Then click ‘Edit’ and select ‘Spaces Only’ from the ‘Tab Policy’ dropdown.
You can further set the indentation and tab size.
For default text editor:
Window->Preferences->General->Editors->Text Editors->Insert spaces for tabs