How can i fix my vauxhall tigra 98 power steering?

My tigra is making a nusty sqeeky sound by the power stearing pump when i turn the car. when I stop the car after a drive it is loosing power stearing fluid by the resevor cap breater hole. it is not leaking any were else but by the breather hole on the resevor cap. I have to top it up all the time i drive it. It also seems there is quit alot of air bubles in the fluid.
How do I fix this problem guys?