Silly question, how do I read the execution plan to know what columns to index?

I created two indexes (just eyeballed query and guessed). The first one seems to have improved the query, but the second one (though being used) still doesn't seem to improve anything.

-- Two indexes created --

CREATE NONCLUSTERED INDEX [IX_tblCustomer_CustomerId_EmailAddress] ON [dbo].[tblCustomer]

(

[CustomerID] ASC,

[EmailAddress] ASC

);

CREATE NONCLUSTERED INDEX [IX_tboWorkOrder_CustomerID_DatabaseInfoId] ON [dbo].[tblWorkOrder]

(

[CustomerID] ASC,

[DatabaseInfoId] ASC

);

Thanks,

Rob