Just don't put anything on the walls that will damage the paint (tape, pins, blu-tack etc) and read your uni's regs re things like fairy lights and changing the curtains (a lot of places won't be happy about either of those things due to fire hazards, but if you leave fairy lights unplugged when you're not there you should be ok).
I'm all for a bit of decoration (I did things like having lots of plants in my room, making my big built in corkboard into something of a photo montage, buying coloured table lamps and colourful bedding, sticking posters to my wardrobe etc), but you definitely need to be careful you don't damage anything.