How to show interest to serve as a TPC or reviewer in top conferences?
I can only answer for computer science, but around here TPCs of conferences are generally by-invitation only. You cannot (officially) apply for it per se - typically, the organizers will form the TPC based mainly on previous members, filling any openings with outstanding members of the community not currently part of the TPC. Usually, these outstanding people are personally known to the organizing committee, so no formal search or something like that is conducted.

Of course, writing to the organizers expressing interest cannot hurt, but my impression is that if you do not already know them, your request is sadly likely to be ignored.

Edit:

I should add that I did not mean to imply that it is all just a matter of "knowing somebody". Seeing that you are still a PhD student, you will likely simply not be high-profile enough at this point to be considered for the TPC of a really good conference (these usually only consist of eminent faculty).