Ok first of all calm down! You'll be fine.
If you dont have your p45 you cant give it to them, just say you are waiting for it from the job centre, you cant do more than that.
Yeah Good morning how can i help is good, or hello how can i help in the afternoon. Arrive about 10 minutes early to make a good impression and so someone isnt waiting for you as they will be waiting from before the alloted time.
You shouldnt run out things to do in a toy store in the run up to christmas as its likely to be rammed which is why you are there, if you do tidy shelves, sweep up or anything else that others do or you are told to do.
In retail you cant just use the toilet whenever you want! You cant usually go in work time, you can only go in breaks so just wait until your breaks. You can always arrive a bit early in the am so you can go before you start work then you should be fine until lunchtime or your first break.
You should smile, i doubt you look like a peado, just a quick smile when greeting the customer and when saying goodbye is all thats needed.
Either shirt will be fine i would think, pick the one that fits best and looks smartest. You absolutely must must must must wear shoes. Tough if you get cramp thats just one of those things working in retail, your feet will absoltely kill you most days when you finish but thats jsut part of the job. What else would you wear anyway? You certainly cannot wear trainers or converse or anything like that.
Overall jsut try and be nice and helpful. Dont moan about anything at all and do everything you are asked to do however daft it seems. Remember in a job you arent equal you have to do whatever your boss says even if they are an idiot.
Good Luck!