Chances are you are using the water that comes out of the faucet in your home. And that's probably fine, but there are a couple of considerations. First, most municipal water sources have chlorine added to them as a purifying agent to kill bacteria and germs. Plants only need chlorine in trace amounts, not in the amounts added to water sources. Fortunately, the solution here is easy. Fill your watering can with water and then let it sit for a few hours to allow the chlorine to dissipate. After that, you are ready to water.
The second consideration is whether or not you have a water softener. Water softeners remove heavy minerals from water but leave behind salts in the process. You can water with softened water in a pinch but using it on a regular basis will build up salts in the soil. Over time, plants won't take nutrients up properly and can show signs of burn on their leaves. If you have a softener, only use water from a bypass valve for regular watering.
A few great sources of water are well water and rainwater. We happen to have a well at our house, and the water is filled with iron and calcium. Bad for our bathroom fixtures, but great for plants, they love the extra minerals. Calcium can leave white marks on leaves, so we are as careful as possible to only water at the roots.
Rainwater is also a really great water source. Capture it in a bucket or rain barrel, for pure water that is really great for plants. And if you can, set some of your water-loving plants outside during a drizzly rainstorm. It will wash the leaves and give them a thorough soaking from Mother Nature.
Finally, if you have sensitive plant babies, unusual plants that are known to be fussy about care, you can use distilled water. There is certainly an extra cost to this, but the purity of the water can be really good for your most prized plants.
You know watering is important and that watering consistently is key. Now make sure you are using water that is good for plants too.