Most folks use what is called a saddle valve. This clamps around a cold water line such as the one that goes up to your sink faucet. When you run the valve down for the first time, it pierces the water line. When you turn the valve back to the open position, water will flow through it. Your supply line to the RO
unit connects to this saddle valve. You need a minimum of about 40 psi water pressure to get good performance from an RO
membrane. Booster pumps are available, but they tend to be pricey. I know the saddle valve sounds like a disaster waiting to happen, but I've never had one fail.