What you need....
1. Patience. A hell of a lot of it. It only takes a few minutes to mix plaster for a couple of molds - you can churn out a mold in about 30 minutes, including drying time. It's just VERY boring after a while...
2. Molds. Some will say to start by getting a couple of them. I say buy a load. The more molds you've got, the less time messing around making your tiles/blocks, the quicker you can actually get building your rooms etc, etc... I suggest getting mainly floor & wall tiles (maybe 2/3 of each) - you will use these the most. The other molds for stuff to adorn your rooms is used a lot less frequent.
2. Good plaster. You can buy cheap plaster of paris - don't. Get dental plaster, crystacast, excalibur, etc. It's more expensive, but much better - it's also heavier, so be careful if you plan on building large buildings.
3. The hirst arts vibrating table. Read their site and make one. Makes life easier.
4. Paint. realistically, you can use any. The 'latex' stuff is good for protecting the cast, and is available in a spray can. I basecoat with this, then use acyrlic paints.
5. More patience. Making the rooms seems easy enough to begin with, but you'll come to realise it's not. It's trial and error really...
That's about it. Grab a cutting mat, some 'aleens tacky glue', craft knife set, pinvice, fine sand/glass-paper and needle files ad you're on your way. For building walls, you'll need some lego as a frame to keep your walls straight...
Hope it helps...