I’ve had it for a few weeks now and watered it once. It was on the windowsill for a couple of weeks but has since been moved to my dressing table which gives it some sunlight. About 5/6 leaves have fallen off & the tips of a few leaves are brown.
This appears to be Dracaena marginata, and as such, it does not appreciate direct sunlight. It tolerates fairly low light conditions, but does better in bright daylight, without sun. If you want to keep it in a window, then a north facing one is probably best, but it doesn't really need to be in a window, just somewhere it gets reasonable daylight and away from heat sources such as radiators.
Assuming it is winter where you are and your home is heated (which might mean a need for more frequent watering than you've been giving it) the usual rules apply - water when the surface of the soil feels dry to the touch, but not so dry the soil has shrunken from the sides of the pot. Water thoroughly, allowing any excess to drain away freely from the bottom of the pot and empty out any outer tray or pot after 30 minutes so the plant is not left sitting in water.
I'm not sure what's caused the minor leaf damage shown in the image - check beneath all leaves, closely, in case your plant has an insect invader of some description, otherwise it looks like physical damage of some sort. Further care instructions here https://homeguides.sfgate.com/indoor-care-dracaena-marginata-42795.html