Need to purchase a Macbook but have absolute no idea about it?

Alright, so recently I got a 400 voucher to purchase something from Apple store, and I decided to go with a laptop or notebook or whatever you called it - the portable one.

A little information about me: I'm a college student, so mostly use it for word and research purposes. I don't play games. I don't listen to music. All I do is watching YouTube, and browse around the internet. I have never used a Macbook in my life, and I'm super not good with IT stuff; I have no idea about memory and stuff.

What I need:
1. A thin and light portable one (I have this 2kg/4.4pounds Lenovo laptop, and I have never brought it to school because it's so heavy!)
2. Can process things fast (my sister have another Lenovo, and when I do my research for assignment, it was slow to the point I almost throw it away).
3. I only have 1k around cash. But if 1k won't get me one that's good enough, I would choose not to buy it and save some money. So max 2.5k budget, but preferably 1k.

Thanks! I would award the best answer with a five stars~

Added (1). OH! And yes, do I have to install Microsoft Office or it would come with the laptop?

Yeah mate get a macbook. I used to have a pc before but when I changed to mac my whole life changed. It's so much better, smoother etc… Just that you can't really play games but games are for playstation anyways! Right now I have a macbook pro 15inch and it's working perfectly. Only thing is that I think it's a bit too big but apart from that it's all good. For you I'd suggest getting the macbook air, they're slim and very light. As you won't be doing heavy work like editing stuff and using softwares like photoshop, the macbook air would be fine. They cost around £850 (about $1300). Finally Microsoft Office won't come installed but you can purchase it or get it for free using torrents. Hope that helped

$ 400 is not going to buy much of a Mac. You might set a 2009 model for that price.