o2 and phone contract

Hello..I ordered a phone on o2 and it also came with the o2 SIM card..my first question is how do I check the sim balance after placing a call?second question does the o2 SIM card come with free data or do I have to activate?and am I paying for the O2 calls or the data?i am confused and how to I check the data balance?third is that I don’t understand my payment because I can see different kind of payments..I would like to know the total amount that they will be deducting from my account monthly both the o2 and the phone I ordered on contract..thank you