i dont know what a bottom bracket does? i need a new one, is it worth spending a lot?
It's the set of bearings at the bottom of your bike to which your crank arms and subsequently pedals are attached. It allows your cranks to rotate smoothly when operating correctly.
Cost depends upon the type of crankset you're running and if you have trouble with reliability from standard units.
What type (or make/model) are your cranks?
If you don't know what it does. How do you know you need a new one?
yeah!! i was told by the company my bike was getting serviced by that the bottom bracket was worn out so i thought i would find out if it was worth spending big on it...
As a general rule - No. The expensive ones only last longer due to maintenance and since it would appear that you dont do that yourself; I wouldn't bother spending big.
Save your cash.