Yes you will need an antenna. In the US, Digital and analog broadcasts use the same antennas, so if your currently getting good analog reception, your current antenna should be good enough.
If you buy a new TV there is about a 99%+ chance that it will have a digital tuner built in to it, so you would not need a converter box; you would just connect your TV antenna to the DTV and have the DTV scan for digital over the air signals.
As to the other 1% these are things to look for:
TVs manufactured before March 2007 may only have an analog tuner. Some of these are labeled "HD ready". If it says "HD ready", it does not have a digital tuner. There is supposed to be a warning sign by the TV in the store if this is the case. There are not too many of these left and Best Buy has already purged these from their stores.
If you have some doubt, ask the sales person to show you the specs for the TV. The digital tuners for over the air signals are called "ATSC tuners". The ATSC should be listed in the TV's specs.
You cannot tell if a TV has a digital tuner by looking at it. You can buy convertional "tube" TVs that have digital tuners and they used to make LCD TVs that only had analog tuners.