First timer! Can anyone help me work out what might be wrong with my 323?

[rvk1986]

I have a 1999 Ford Laser, which is apparently exactly the same as a Mazda 323. It's fuel injected, 5 spd manual. It looks like this http://en.wikipedia.org/wiki/Image:M...III_Silver.jpg except with Ford badge.
My problem/s is this: Sometimes after i have been idling for a little while, waiting for the lights to change or something, when I go to take off it has hardly any power, it goes really really slow. It also has a rattling sound in the engine when is at low revs, like 1200rpm or under. It sounds like there is a little piece of metal rattling about in there. It's also using more fuel than it should. I'm so annoyed because I only bought this car two weeks ago! Grrr... Any help would be much appreciated, Thanks
PS I know little to nothing about cars so you might have to explain things in layman's terms, Thanks!