What do I need to provide

[Kat1924]

I have been out of work for 2yrs due to an illness so was on tax credits and esa.
I have been offered a part time job in January just working in a local family run small shop for 16hrs on min wage and on a trial 3mth period because they know of my illness and want to see how I cope.

I know I will have to inform esa and loose that benefit, but when I ring tax credits up what do I have to provide them?? Do I just give them the start date, hours I will be working and what I will be earning or will they want me to send in a contract (won't get a contract as it's just a little shop and they just want to test me out for 3mths) and is there anything else I have to provide to tax credits??