Using the internet, using apps like FaceTime, using email etc all use data.
Data is normally 'free' when connected to wifi. Data normally costs when connected to the mobile network via 3G.
O2 data costs are . They are €1 per day for any usage up to 50 MB then €1 per MB. You can also buy a bundle of data of up to 700 MB data for 30 days for €10. O2 are expensive for data.
You might be best off checking out other providers such as Three who give unlimited data for 30 days every time you top up by €20, unlimited texts, unlimited weekend calls and €20 credit to spend.
Do you want a phone with a keyboard? or are you happy with a touch screen keyboard?