Playing out a tragic hero in D&D is a very difficult role playing challenge, because D&D ultimately gives you very little control over your long term fate. It also gives only limited control to an individual player to choose themes that will come up in play. "Tragic Hero" involves a story arc, and D&D very much has a game's perspective on play in that it's linear and you can't know what is going to happen next.
That said, I'm always for players and groups trying to up the artisanship of their play, and "Tragic Hero" while a high challenge, is a good one to set for yourself.
The attributes of a tragic hero is that he desires something which is good, and possesses many virtues that make him admirable, but has some character flaw that leaves him blind to the reality of the situation or which ensures that he'll always end up taking a low road to his destination. In other words, his means will ultimately undermine his ends.
There are way too many ways to go with this for me to give you firm suggestions, and more importantly, you should conceive the character in a way that is tied firmly to the setting and to the story the DM has in mind. Work with your DM on this. Communicate your desires, and ask for suggestions as to what sort of goals and flaws might work well in the conflict he's envisioning.
That said, try answering the following questions:
a) What does he want which, in his circumstances, a 'reader' might find sympathetic?
b) If what he wants isn't something which would normally seem noble, what background does he has that might make his ambition seem more sympathetic. For example, a person who covets wealth, is more sympathetic if they come from a background of abject poverty. You can still use background to increase sympathy even if what he wants is noble. For example, although he wasn't quite up to the task of writing the grand story he conceived, George Lucas's instinct to make Anakin Skywalker have the background of a slave is perfect for a character that wants most of all to see a strong, just, and righteous government rather than one that pretends to have high ideas, but which is actually too weak to enforce those ideas and too corrupt to live up to those ideas.
c) What flaw does the character have in his moral character, that will lead him to undermine his own desires? Hubris is of course the classic flaw, but you can get more specific.
