It's fairly simple to begin carbonating things; here's a list of the things you'll need to follow this instructable step by step:
-CO2 supply: This can vary greatly between setups. I use a 20# tank, but you can use nearly any size CO2 tank with a regulator. Heck, I've even used 9-20oz paintball tanks to carbonate things! You can find good used 20# tanks on eBay (I got mine for around $40)
-A Regulator -this is a good one.
-To carbonate liquids, you'll need an empty 3L soda bottle.
-A clamp-in schrader valve for the 3L soda bottle - you can find these at nearly any auto parts store. Here's one that will work. Here's another.
-To carbonate fruits and other objects that you can't pour, you'll need to build a pressure vessel out of PVC pipe and other standard fittings that can be found any hardware store. This will be discussed in detail later in the instructable.
Step 1: Setting up your carbonating equipment
Twenty pounds of CO2 can carbonate a lot of water. Remember that no matter what you're carbonating, it is still CO2 dissolving into water (this is why high-water-content foods work best). A 20# tank holds enough CO2 to highly carbonate around 500 gallons of water - with this amount you could drink 2-3 liters of home-brewed seltzer per day and not have to refill the tank for several years! The CO2 cost of what I make in this instructable is only a few cents!
In addition to the tank, you'll need a suitable regulator to bring the gas down to working pressures of under 100psi. I use a regulator that allows me to adjust between 0-100 psi, pressure which are more than suitable for carbonating things. You can find a good regulator at any home brewing store. Ideally it should have two pressure gauges (one for the high-pressure side and one for the low-pressure regulated side), a safety relief valve, and some sort of shutoff valve.
Note: Please DO NOT try to use a tank without a regulator - CO2 is stored in a cryogenic liquid state in the tank at pressures of around 1000psi and opening the tank valve without a regulator attached could lead to a very dangerous situation! I suggest that you read and become familiar with the MSDS sheets for CO2 - it's a very safe gas to work with as long as some basic procedures are followed.