Firstly, the fact that you are 'desperate for a child' does not necessarily appeal to Social Services as an ideal attribute for a foster carer, but i'm sure there is much more to you as a person than that. It is much more important to put across what YOU can offer a child/children as a carer. Also, - and i'm not suggesting this as a general overview, just in MY experience in Social Services - many children are deprived, disadvantaged, delinquent and have many, many problems and issues arising from their lack of parenting/discipline, such as drug problems, extreme behaviour patterns etc. so you will need to demonstrate an understanding of this and of the patience you will no doubt require for such a position. You will be asked about your own upbringing and your thoughts on this and your intimate relationships/partnerships, and how you have dealt with issues etc.......it is a very in depth process. Any information you give about yourself/family will be checked out thoroughly. I realise that this sounds all doom and gloom its just best you are aware of the ins and outs and i dont know all of them but i'm sure you can understand why it is necessary, and on the other hand if you are not put off by any of this it can be a very rewarding role.
Hope this helps and good luck.