It really depends on how much speed you've got and how much experience - in this case its your first urban so the jump size is up to you and whatever you feel most comfortable with.
People will bitch and say its not urban if its not less than half the height of the rail- being that its your first time hitting urban I'd honestly suggest a lip that is half the height of the rail, or slightly higher. Urban is a whole different game and it takes some time to get used to it. Make sure you've got ample speed, make sure the first rail you hit isn't too long and is either flat or down. Build your jump flatter- don't think you need a lot of kick to get onto the rail because it'll more than likely scare the piss out of you if you're way above the rail.
You will not need wax, but inspect whatever rail you've chosen. Is it fairly short? Does the rail itself have any burrs in it? Make sure the rail isn't aluminum, look at how much spacing you've got (does the rail end with 5 feet before a wall? you probably wouldn't want to hit that for your first urban). Lastly check the consequences - does the rail drop off to flat on one side, or is it sloped like the staircase?