how can i talk to him beside letters

Very occasionally he does fan/autograph signing events, and whenever he does a play, he signs autographs for fãs outside the stage door. Apart from those (where you'd only be able to have a few segundos with him) there really is no way fãs can get to talk to him.

As far as I know, David Tennant doesn't have an e-mail account that is opoened to the public.

Neither is his phone number. LOL!

Well, depending on what country you are in, if he happenes to be a guest or guest dj on a radio program were you can call in and speak with him.

Or, you could try yo contact any studio or go through his agent c'mon! all actors have an agent.

but otherwise I don't have a clue.