Answer: It is a bit difficult to provide instructions without knowing what kind of tree it is. If it is a dwarf spruce or another hardy evergreen, you will have a very hard time keeping it healthy indoors because these trees need a winter chilling period in order to meet their natural requirements for growth and overall health.
These trees are often used as cut holiday trees, too, so even if not all that healthy it should look good for quite some time. To help the tree stay presentable for as long as possible you will need to keep it out of warm drafts, keep it in bright indirect light, maintain a high humidity level and keep the soil moist but not sopping wet so the tree does not dry out. A cooler room temperature is also better for keeping the tree healthier longer. However, eventually the tree will fail to thrive and will be (sadly) best discarded.
If it is a Norfolk Island Pine (another tree often sold as an indoor holiday tree) you will need to keep it in bright indirect light but with cool temperatures of about 55 degrees in the winter and then average home temperatures in the summer. The tree appreciates ample humidity and the soil should be kept moist but not sopping wet. Again, avoid drafts. This tree is slow growing and very long lived as a house plant.
Good luck with your tree.
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