I am almost replacing the steps to my front porch. It is 5 steps so I definitly need a handrail, but I don't know exactly how high to make it. I have some posts that should be plenty tall but I can attach the handrail at any point along the post. I assume there is some standard height from the stair tread that is usualy used in the US.
The standard is usually 34" to 38". Check your local building codes - apparently some cities specify exactly 36".
The International Residential Code covers just about everything you could want to know about stair standards.