You would want this to be done server-side. So the user within the app
would configure what price they want the trigger to happen at, and you
would send this information to your server. Then your server would send
them a push notification when this condition is met. You would not want the
app doing frequent requests to the server, or be kept running constantly in
There are services such as Parse and Urban Airship that can handle the
push notifications side of things for you. You'll have to set up other
stuff in order to be able to determine when to send them and such, but
these services will help you.
I would recommend finding tutorials on PHP, since that would be useful
to know. But also you should find tutorials on integrating and using Parse
and Urban Airship (I would probably recommend Parse over Urban Airship, but
it's your choice).