I'd recommend Warlord difficulty if you can't remember the game well. Chieftain is just ridiculously easy. I think my first CivIII game was on Chieftain, and it wasn't really a challenge.
Standard map, I reckon, or even Small. A Large map game takes ages, and in this first game you'll be getting the hang of things again, probably realising lots of things you could have done better. Long games can be a pain even if you find you're winning easily.
Make sure you give yourself lots of land: personally I prefer Continents or Pangaea landmass-type, as Archipelago can give you a frustrating small-island start, with the possibility of no trade being allowed until your ship-technology progresses. Independent of the landmass-type is the proportion of land to sea. The highest sea (thus lowest land) settings are IMHO for specialists looking for a big challenge.
Number of opponents: I always go for the max for the map. Means more rivals, but on the other hand more trading opportunities. Back in Civ2 I could be a science-powerhouse on my own; in CivIII trading techs is a huge (and very satisfying) part of the game. With a low number of rivals, you're more likely to grow quite a bit before running into rivals - but on the other hand your tech will go slowly because you won't meet anyone.
Age/temperature etc: I like the middle Age setting, or a young planet. Hills and mountains are good for production. Warm and Wet gives you loads of Jungle and Marsh, which is very frustrating. Try middle settings on all these, see how you like it.
Another important setting is the allowed victory types. If you like wiping all opponents off the face of the earth, make sure "Domination victory" is unticked. Domination makes you win when you control X% (I forget the number, 65%. 70%? - never use it) of the land.
Culture-flips is IMHO a great part of CivIII. You can switch this on or off. On means a captured city has a chance of "flipping" back to its original owner if they have more culture than you. Some like it, some don't.
Enjoy! And this forum is packed with ideas, solutions to common problems people run into - [EDIT] Welcome!