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 fans outside the stage door. Apart from those (where you'd only be able to have a few seconden with him) there really is no way fans 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 u are in, if he happenes to be a guest of guest dj on a radio program were u can call in and speak with him.

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

but otherwise I don't have a clue.