What you need to start fragging
Step 1: Download the latest game update: Point Release 1.32 and 1.32c. 1.32c is a small security patch that is not required, but recommended. Install 1.32c after installing 1.32.
Step 2: Download a 3rd party server browser to find and join games. Some of the popular ones are The All-Seeing Eye and Qtracker
Step 3: Get some mods! Modifications must be unzipped to your Quake III Arena folder. Here is a small sample of the more popular q3 mods. CPMA and OSP are pretty much standard now days, so it's recommended that you at least download those.
CPMA Adds competitive features, gamemodes, maps, and bug fixes to the standard Quake 3 in "vanilla" mode. Also includes "promode" that changes the physics, weapons, and armor system.
OSP is no longer being updated, and has been merged with CPMA. Many still prefer this version over CPMA, as CPMA fixes the 125 fps "bug" making the movement feel different in the vanilla game. Like CPMA, it too offers the competitive features that have become standard, along with additional gameplay modes such as instagib, and Clan Arena.
Rocket Arena 3
The premise behind Rocket Arena 3 is that players start with all weapons, and lots of ammo, but are unable to collect items, and respawn. The focus is on pure combat rather than map control. Players spawn in a lobby area, and must choose an arena upon joining the game, which are small maps within a larger map, and can contain their own rule set. The primary modes are Duel, in which 2 players face off until one is defeated, and Clan Arena, in which teams of players face off until a team is defeated.
Threewave is a Capture the Flag mod that adds a variety of modes, and maps to the standard CTF formula.
Excessive is a simple mod that changes the weapons behavior, making them greatly over-powered. Excessive Plus is an updated version based on the Original Excessive Overkill mod by Mr. Pants.
Step 4: Keep the game fresh with custom maps! Custom map .pk3 files go in your Quake III Arena/baseq3 folder. This step isn't required, however some servers run custom maps that you'll need in order to join. If you don't have a map that a server is running, and you're having problems with auto-download, check the map name in your server browser, and google it in your web browser. The Threewave map packs are highly recommended if you're a CTF fan.