Hi...
I am trying to interface ultra sonic sensors to the car
assistance system for finding the distance of an object from the
car...or there is any other alternative???
Have you done the google homework ? Ultra-sonic sensors are fairly common in robotics projects for this purpose.
Have read few posts from you and few other, students… try to understand this, people will only help you, when you prove sufficiently well that you have tried hard enough on your own merit, and when stuck at a specific point, and give very specific inputs, as to what is not working.
Google for the term “homework question”, and you’d see what I mean.
Are you sure you’d need a beagleboard for your application?
Simply examine the capabilities of the BeagleBoard, Learn it and
understand it.
Google the type peripherals you are interested in using, they most
likely
come in many variations.
Think how these devices might interface to the BeagleBoard and how
you would write your software to utilise them.
Be creative 
And there are alternatives to most any situation.
djlewis