* Furniture-painting should be something you do if you're pretty experienced IMO. Many DIY-jobs just look like....
* Professional result come not so much from the paint itself, but the prep, techniques, and equipment!!
* Call around to some painters, cabinet-shops, or maybe even autobody-shops.
* Good prep to do ahead-of-time would include cleaning & a thorough scuff-sanding with ~100-grit paper.
* There is just no way to duplicate a good sprayed finish on GOOD furniture if you're a rookie.
* Priming, and possibly sanding a couple fully dried prime-coats with ~150 to 220-grit can be very helpful if finer finishes are desired.

>>> Clean & Dulled are crucial!!

What kind of pieces are they?
Are they daily-use?
My above points aren't as crucial if the pieces are just knock-around items in, say, a kids playroom, or casual outdoor chairs.

Faron

But to answer your question, use paint thinner with oil-based paint and water with latex. What type of furniture will you be refinishing/painting? If you are painting a piece that normally comes in contact with grease, use oil-based paint. Otherwise, latex is much easier to use and clean up afterward. I've done a number of pieces myself, including kitchen cabinets, dressers, tables/chairs, beds, desks, and bathroom vanities. Since spray-painting is better done by a professional (and costs $$$), I would recommend a high-quality paint brush if you choose to do the job yourself.