cutting objects?

i want to make an arc whats the best way?

this is how i am making: i made a circle...(with stroke only)
now i want to cut some circle so i am left with an arc....but how do i cut it??
just like there is a knife tool in Fireworks...wht bout?

Press "M" to select your square marquee tool. (If the square isn't selected, Hold Shift and Press "M" to cycle thru the different selection shapes) then draw a square thru the middle of the circle. Now just press delete. And that should be all. Just make sure you are editing the layer that has the circle on it.

i have been trying that it gives and error
Warning no more than 50 % px are selected
no edges are selected....?

Sounds like you are on the wrong layer. Make sure the box selection is bigger than the circle layer... make sure the selection is not feathered...