The only way you can guarantee sitting together is to pay for the pre-selected seating or group seating options. (Pre-selected seating is where you choose the actual seats; I think it costs £20 per person. Group seating is where the airline will seat you together but you cannot choose where; that costs a bit less). If you're not prepared to pay the extra then I'm afraid you will have to just take whichever seats you're given at check-in. If a significant number of people have pre-selected seats, or if a lot of people have checked in before you, then you may be spread out over the entire plane.