Can i and will i have to change anything on my phone and how much will it cost me to make calls/messages?
Your phone should be OK to use. Make sure you have credit, as goodybags are for use from within the UK only.
Roaming in other countries
Making calls and calling your voicemail (charging in per second increments after the first 30 seconds) £1/min.
Receiving calls (charged in per minute increments) £1/min.
Sending texts 30p
Receiving texts Free
Picture messages (MMS): Wherever you are in the world, sending an (MMS) to any UK mobile number (wherever it may be in the world) (max 300KB) costs 16p, and sending to a non UK registered number costs 24p. Downloading MMS is totally free.
Note: Data must be enabled to use MMS but no extra data charge is made for using MMS.
Data Roaming £5.00/MB
You may find MMS more useful there, take a look here: http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/MMS-costs-SIMPLY/m-p/1478209/message-uid/1478209#U1478209