new techno-challenged person seeking help

Wow, seems like there are lots of people here who know what they're doing. So this could really be the place for me to come or it could just mean I'm way out of my depth!
Anyway, I have just taken the step of getting a computer at home (yes, I am THAT green).
I have a Macbook and I connected to an internet provider here in Australia who could only offer me a wireless connection. It's a little plastic thing that plugs into the side of my computer & that's all. Nothing else plugged into the wall or anything.
When i signed up the internet provider told me that there was a strong signal where I live.
However, I've been really disappointed at the speed of my connection, especially at night. Occasionally the connection drops out but the ongoing problem is that it's sooo slow. It can't even sustain a Skype call without losing the call/chat constantly.
My provider said it was probably down to congestion but noone else I know who has wireless broadband seems to have my problems.
I have a plan that allows 5GB downloads per month (is that even relevant). I've tried using it in different parts of the house and even in the best part it's painfully slow and hopeless at night. (mornings are OK but still not wonderful).
So my question is - is there anything i can do about it or do I just have to put up with it? I was hoping there was some sort of booster equipment I could buy to make it better.
Can anyone out there help?