Though there are plenty of gadgets on the market for home-made soda, to carbonate water you really only need a couple of things: a seltzer bottle (also called a soda syphon) and chargers.
The seltzer bottle holds regular water, with typical capacities of 1 or 2 litres. These look alot like thermos bottles with a spray mechanism built into the lid. The chargers do the carbonating work. Soda chargers are small cannisters of pressurized CO2 that come in either snap-on or screw-in varieties (check the bottle before buying chargers). The charger fits into the spray head. Then all you need to do is press the plunger, and freshly carbonated sparkling water is dispensed.
You'll usually use 1 charger for 1 litre (quart) of carbonated water, and they come in boxes of 10. Depending on your bottle, that works out to less than buying club soda from the store. Though you'll find that homemade seltzer isn't quite as fizzy as store-bought.
The cost for seltzer bottles can range dramatically, based mostly on the materials of the bottle (aluminum, glass, stainless steel) and the durability of the spray head mechanism. Cheap soda syphons with plastic parts will not last you very long and aren't worth buying.