Starting at the gym. need some cardio. how much treadmill (brisk walking) should i aim for. 1/2 mile?

Depends: It depends on how conditioned or de conditioned you start. Would suggest stopping whenever feeling fatigued or with any symptoms- pain, shortness of breath, etc. otherwise aim for 15-20 min and increase slowly.

.5 mile is a good st: .5 mile is a good start

Start slow: I presume that you have not been walking regularly before this. I recommend starting at one mile first, then increase half of a mile every week until the goal of 3 miles a day, which is about 10000 steps. Make sure that you are wearing properly fitted shoes with good insoles to make your walk more enjoyable.