Advice for sound system, Best Buy? Options

[A_N_T_H_O_N_Y]

I'm looking to upgrade the sound system in my '95 GT. Right now it's all stock (I think) except for an old aftermarket deck that came installed in my car when I bought it. I definitely want some subs, and a deck, but I'm not sure what else I would need. I've never bought a car audio setup before, so this is all new to me. Is it worth it to change the stock speakers (my car has the two rear, two front door, and two tweeters)? I'm not looking to spend an excessive amount of money. I live in Canada, and we don't really get very good deals here. I know that general consensus around here is to stay away from places like Future Shop and Best Buy, but I have a cousin that works at Best Buy, and he told me that he can get me a significant discount on anything car audio there, since all of their stuff is marked up massively.

So yeah, any advice?