whatever you ar comfortable with, really. I hardly ever do urban, but if it looks like you can clean it, try it anyway. I usually build a really small jump or not much of a jump and try to ollie as much as possible.
try and find something that isnt to high off the ground. on my second urban i did, i made this mistake and ended up with a pretty bad injury. i gotta broken tailbone and a sprained knee and the inside of my leg has like 3 huge bruises. just dont get to pumped for something to intense.
The second rail I ever hit was a wooden 33 stair, but that's because we didn't have a park. A 15-ish stair with a natural drop in would probably be ideal. If there's a natural drop in you can focus on the rail instead of on getting speed.
my first was a nice wooden down rail about 25 stairs, don't be afraid to build a bicker kicker for your first rail as long as you feel comfortable, and try to find on that has a nice in/out run and that is low confidence....and as quebecfreestyler said wooden stairs are a plus because if u come off stair side they are very easy to ride out