How do I obtain the document id in order to delete from index?

[Enrique Lamas]

Hi,
I've been reading Lucene documentation and I see there are two ways of deleting 
a document from an index: by id and by term.

Supose I have an index with three fields: field1, field2 and field3, and I want 
to delete all documents with field1=value1 and field2=value2.
I think I must use deletion by Id, but I don't know how to obtain the Id of the 
documents I want to delete.

Thanks