The first thing you need to make sure you have before you bring home your fresh-cut Christmas tree is a tree stand that has an ample reservoir in it for water (via Home Depot). It's also a good idea to keep a spray bottle on hand to lightly sprits the needles here and there, as the trunk isn't the only part of the tree that needs water to remain moisturized and healthy. If you have trouble keeping track of how often the tree needs watering, there are automatic tree waterers that you could buy, some that simply alert you when the tree is getting dry so you can water it, and others that are actually equipped with a tubing system to do the watering for you. 

If, however, you just want to do this the good old fashioned way, then simply check the trunk of your tree daily to ensure that there is ample water in the stand. When the tree is new, it should suck up a good deal of water. As it nears the end of its life, it will take in less water, which is when you are likely to notice the needles turning brown or falling off.