how can i write a node to subscribe to the “joint 0”？

edit

You have a single topic called /joint_states that has, presumbably, sensor_msgs/JointState messages being published on it. I'm unsure if you are trying to say that a single message published on the topic contains information on all 6 joints (i.e. the name field would have 6 entries in it, which is pretty common), or if you are saying that every 6th message published on the topic is for joint0. Either way, as far as I know, it is not possible to filter topics at the subscriber level like you are interested in doing. I can think of two easy ways of working around this: (i) pulling out just the information for joint0 in your callback (if there is no joint0 info, just exit the callback), or (ii) writing an intermediate node that subscribes to /joint_states, filters out the joint0 information, and then publishes just that info on a separate topic.

EDIT

Here's a simple example in Python to illustrate the concept:

#!/usr/bin/env python
import rospy
from sensor_msgs.msg import JointState
from std_msgs.msg import Float64

def joint_states_callback(message):
    # filter out joint0 position:
    for i,name in enumerate(message.name):
        if name == "joint0":
            pos = message.position[i]
            pub.publish(pos)
    return

if __name__ == '__main__':
    rospy.init_node("example_repub")
    pub = rospy.Publisher("joint0_topic", Float64, queue_size=1)
    rospy.Subscriber("joint_states", JointState, joint_states_callback, queue_size=1)
    rospy.spin()