You won't need a lot of specialised "dollhouse" tools to get going in this hobby. Most of this stuff you probably already have, if you've done any other woodwork or home repair. If not, it's easy to find and not very expensive.
Get yourself a small toolbox or container to hold your tools and keep them clean and organized. Try to prevent family or friends from "borrowing" your miniatures stuff and loosing it. (I have to hide my craft knives or they're gone whenever I want to work) Label your miniatures tools with tape or paint if you have to.
The absolute minimum you need is sandpaper, glue, a straight edge, and a craft knife. That's all. With that you can assemble almost any house or furniture kit.
when you're ready to get more involved
Here is a list of supplies that will make the projects a little easier to finish. You should be able to find most of these items already in your home, or at an average hardware store. They are common, inexpensive tools, and you will probably want to purchase them as you need them.
sandpaper -- various sizes for rough sanding and fine finish work
steel wool -- better for smoothing final coats of paint and stain than sandpaper
craft or hobby knife (aka Exacto)-- for cutting everything from wood trim to wallpaper, scoring, scraping, picking and pointing everything else. Don't forget the extra blades. There are also some new ones with rubberized grips that make handling them a little easier.
cutting mat -- not an absolute necessity, but the gridded smooth surface is very nice to have under that cutting blade when trimming wallpaper and delicate trim. You can find these in sewing stores, too.
paint brushes -- all sizes, get a pack of assorted smaller sizes, #4 for fine detailing, larger sizes for applying glue, paint, stain, sealer, etc. as well as dusting corners and various surfaces. Get a 1-1.5" house brush for doing larger areas, such as exterior surfaces and floors. It's also good for sweeping dust after sanding. Keep one set of brushes for using oil-based stains and finishes, and a set for water-based paints and stuff. Clean your brushes and they will last a long time.
engineering ruler -- this is a 3-sided ruler marked with various scales on the different sides. Good for measuring plain old inches and centimeters, as well as 1:12 or 1:3 reductions (and other scales).
clamps and weights -- find the smallest clamps you can. There are tiny metal C-clamps out there, and Exacto makes a set of blue plastic clamps that work well enough for holding bits of house or furniture. Radio Shack has some nice small smooth jawed clamps used for electronics but perfect for holding small parts together. Rubber bands also work well for some jobs. Weights can be anything, small bottles of paint, smooth rocks from the beach, cans of stain, bricks...
tape -- for masking off edges when painting, or holding pieces temporarily while the glue dries.
straight edge -- not a ruler, though it may be marked as such, this should be a heavy metal or plastic piece which you can use to score perfectly straight edges along.
plastic containers -- old food tubs, cheap bowls, cracked coffee mugs, baby food jars, etc. You'll use them for everything from holding small bits to mixing paint or wetting brushes. They can be disposable or not, but you will be using them for chemicals so you shouldn't mix them with things you eat off of.
glue -- plain old white glue or school glue is fine for many jobs. Wood glue, obviously, for larger construction projects. Contact cement may come in handy for some jobs, as will hot glue.
paints and stains -- can be special colors you picked just for this house project, or leftovers from your garage. In any case, they should be in good condition and not dried out or gummy. Use the same stains that you would on any other wood when you do minis. Interior latex works well for miniature houses, small bottles of acrylic craft paints are best used on trim or furniture pieces. A tack cloth is a very good idea; they're cheap and great for cleaning wood prior to staining it.
square -- A small rafter square (a triangle device that can sand by itself) helps keep walls square, and L brackets to do the same on the outside walls. A miniature level also can help keep the walls and floors true.
toothpicks --Tooth picks for gluing, and flat toothpics for shims and filling gaps in the wood are also extremely useful.
for more advanced work
There are a number of nifty tools that will enable you to become a master of miniatures, creating your own custom pieces and detailing. This is where the cost can really start to climb, and you will probably need a dedicated workshop to devote to the hobby.
Check out our article on Power Tools.