help!

hey, I need to go into a store to return a pair of pj's that are too small, I don't normally do this sort of thing myself as it is too stressful, I normally get my parents to do it for me, however both of my parents are working today and the window to return them expires tomorrow. I have no idea what to say or do, how long it will take or anything! if someone could please provide like a general script, giving me an idea of where/who I need to go to, what I need to say, the kind of stuff the shop person might say, and roughly how long this is interaction is likely to take, I would really really appreciate it! I have set it in my mind I will be going down to Home Bargains to return the pj's at about 11:45am today (London Time), but it's causing me so much anxiety! Please Help!