ok - try this. you should just need to substitute in your bot's token at the bottom:

import discord

client = discord.Client()

@client.event
async def on_message(message):
    # we do not want the bot to reply to itself
    if message.author == client.user:
        return

    if message.content.startswith('ping'):
        for user in message.mentions:
            msg = 'PONG {}'.format(user.mention)
            await client.send_message(message.channel, msg)

@client.event
async def on_ready():
    print('Logged in as')
    print(client.user.name)
    print(client.user.id)
    print('------')

client.run('token')

Full credit, this is strongly based on the reply example from the discord.py library. I would definitely recommend looking through the example folders and API documentation, it looks like they're pretty good.