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.
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.