Write notes on everything.
It's so handy to have a little notebook to jot notes down here and there. I normally write out to do lists everyday so I can make sure I meet my deadlines, the tasks I've been doing so I can offload it onto my CV and of course general notes you need for phone calls and emails.
A majority of the time your colleagues will be going for drinks or even just to the shops to buy a packet of crisps, make sure you get involved and go out with them! I know it's easy to shy away in the corner and keep yourself to yourself, but if you're interning there for a while then you want to be able to get on with everyone and feel comfortable! It also means you can get to know them better and maybe you'll find common interests!
I guarantee you will make the worst impression if you sit there and do nothing. Remember why you're interning, to learn, to work, to see how the industry works? Not to sit there on your arse doing nothing! If there's nothing for you to do then a good tip is to check out what's on their hard-drive. This way you can get to learn about the company more which is never a bad thing, but be careful in case you accidentally delete something - from personal experience I wouldn't recommend it!
It's really important that you find out about the financial side of the internship. Are you being paid? Does the company pay for your travel? How much does it cost for your travel? It's definitely worth having a sit down and thinking about all these questions. I've had to pass on an internship I desperately wanted to do just because financially it wasn't possible, it really can be a bummer.
Get a good chair.
I seriously wish someone had told me this before I first started interning because despite seeming silly, it's really important especially in an office job. Make sure your chair is comfy and not too big because I can promise you after a few days your back will be killing you. Even if you can't change your chair then just bring a cushion in or anything to make it more bearable, after all you will be sitting in it all day everyday for the next however many weeks!