How can I tell if my brakes need to be replaced?

I own a 2004 Pontiac Vibe. It is the base model and an automatic. I bought it used in February 2005 so I don't know when the brakes have been serviced last.

Lately I've noticed that there is a squeeking noise when I brake. I don't want to take it to the shop because I don't want them to tell me that they need to be replaced ASAP when in reality it's not that bad.

I'll start to slow down but the squeeking noise doesn't appear until right before I fully stop. It also makes a "thud" noise right when it's stopping. I have to press down on the brake pedal quite a bit before stopping.

Do I get the whole ensemble replaced or just the pads? How can I tell what needs to be done without paying an arm and a leg?

Is there anything I need to buy to put on the brakes? Cleaners or anything?

Any help would be greatly appreciated! Thank you.