What sort of budget?
At the cheaper level you would probably end up looking at the MacBook Air but if you can afford it I'd recommend looking at the MacBook Pro as it's a bit more powerful (not super important for your usage) but also has a far better screen which makes a huge difference to the experience when using the machine. Pretty much any current Apple laptop is very thin and light so you won't have a problem in that respect although I'd stick to 13" or 11" ones from the portability perspective.
Also, if you are a student, make sure you take advantage of Apple's student discounts as these are pretty hefty.