GPS Tracking

Hello guys, I am just beginning to play around with Arduino and I want to build a GPS tracking device using Arduino Mega and a GPS shield.
So I want the Arduino to obtain the longitude and latitude of a GPS enabled device (for example an iPhone) and the Arduino itself. But I am really new to this so could someone give me some guidance on how to go about this? I am just doing this for fun so budget is not an issue.

Good that dollars are not an issue. But what about space and battery power budget? A Mega is pretty mega so you're not building a small thing.

Do you really need that big of a part for this? I think a smaller 8-bit AVR part could do the same calculations.
Perhaps a '1284 if you need 2 serial ports.

So I want the Arduino to obtain the longitude and altitude of a GPS enabled device (for example an iPhone)

Altitude?

A GPS attached to an Arduino will be useless for getting the GPS data from an iPhone.

To get the location of the iPhone to the Arduino you will need to write an IOS application and connect to the Arduino using Bluetooth Low Energy (BLE) or WiFi. Time to learn IOS programming!