Transform.up  

// Set's the rigidbody velocity to be
// along the green axis of the transform
rigidbody.velocity = transform.up * 10;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
rigidbody.velocity = transform.up * 10;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
rigidbody.velocity = (transform.up * 10)