Once a Christmas tree is cut, the branches fold in and the cut trunk will seal. Once you have the tree home, before you put it in water, cut off 2-3 cms of the base to make a fresh cut to break the seal so it can take in water. This is very important. It doesn't have to be pretty, drill a few holes, cut or hack it any way you can. If it was a flower you'd crush the bottom of the stem. If you don't do this the tree will not take on water and will die quickly. Once it's in water, the branches will start to come out again, and it's ready to decorate.
The Christmas tree will drink a lot of water the first few days so keep it topped up. Don't let the water dry out, or the base will seal again and you'll have to take out the tree to cut the base again. Some people suggest an aspirin in the water helps, or even a drop of bleach, but we've always relied on keeping the water fresh and our tree is always green even when it's taken down in the New Year.
Christmas Trees, like flowers, will stay fresh looking for about 3-4 weeks. They can last longer but it's the same as plants. If you planted 2-3 flowers or seedlings in the garden, after 3 weeks they would be at different heights. Same soil, same conditions, some are just healthier. It's critical that the trees have fresh water, and it is topped up every day or so. If the water runs out, no amount of topping up will recover the tree, it is damaged, and in survival mode. If your tree cannot take on water it will die, become brittle and then it is a fire hazard with the lights on. Do not let your tree dry out, and if it's not taking water every day, cut the base more.
It's nice to have your tree near the window, but direct sunlight will be more damaging than a shady, cool corner. It will handle plenty of decorations, but try to limit the lights to evenings.