If you are being paid and you are looking after the child for more than two hours a day, then you need to be a registered childminder with Ofsted. It is a lengthy process which includes undertaking training courses and keeping up-to-date records, etc. If it is for less than two hours a day then you don't need to be registered.
If you go ahead with looking after this child, depending on what hours you're required to do, it's upto you whether you charge by the hour or per session if it's a regular arrangement.
Depending on where you live, the charges can vary from �2.50 upto �4.50 an hour. You could go to the NCMA website (National Childmiding Association) at www.ncma.org.uk - this will give you all the information you need. Good luck.