How to get the change sets for a certain file

There is a getBlame method on com.ibm.team.filesystem.client.IFileSystemView.

IFileSystemManager manager = FileSystemCore.getFileSystemManager(workspaceConnection.teamRepository());
IFileSystemView fsView = manager.getFileSystemView(workspaceConnection);
List results = fsView.getBlame(component, fileItem, null, progressMonitor); 

In the list, entry n is the IChangeSet responsible for producing line n in the file.  (line numbering starts at 0).

How can I get the workspace handle so that I can get the workspaceConnection object and what is component and how do I get it?

You can find an ISharingDescriptor which will give you the workspace handle and component handle.

ISharingManager manager = FileSystemCore.getSharingManager();
IShareable shareable = manager.findShareable(new PathLocation(relativePath), ResourceType.FILE);
ISharingDescriptor desc = shareable.getShare(progressMonitor).getSharingDescriptor();
IComponentHandle component = desc.getComponent();
IContextHandle workspace = desc.getConnectionHandle();  //should be instanceof IWorkspaceHandle