You need to be a qualified nursery nurse to become a nanny ideally. Some parents would consider an unqualified person if you had alot of experience but are very unlikely to consider you with neither.
You will need to go to college full time to do either a BTEC National Diploma or the CACHE Diploma. Both courses are for 2 years and you will need at least 4 GCSE grades of Cor above to be accepted, if not you will have to do an introductory course first. Another thing you could consider is seeking employment in a nursery as a nursery assistant and gaining an NVQ 2 & 3 while you work. You will get paid but it is hard work to work and study at the same time. I suggest you contact your local college for further information.