Do have a look through other posts on this board about employing a nanny.
A recent reply I wrote which has some things to think about in terms of costs is on London Nanny Costs thread.
I will need childcare for my two children. I think a nanny would be the best option
Why do you feel it would be the best option? Are you needing an early start time, a late finish time? Do your children have many activities they need to go to, will that change over time? How old are your children, will things change once oldest goes to school?
A nanny may be a good solution but it may be hard to find. Your location could make a big difference to things, such as salary (as it does vary from city to rural locations) and how you go about recruiting someone. You may need to consider how close you are to public transport, you may need to consider how far someone travels to you - where would they live to be able to get to you easily?
I would be so grateful for any help in paying the nanny
As others have said, agree a gross salary. Do your budget, make sure you calculate annual cost - "a nanny is not just for when they work but for all year round". Calculate minimum holiday entitlement and work out what holiday entitlement you would offer - do you need to calculate in hours or days? Hours is often best, especially if working hours each day vary. Minimum entitlement is 5.6 x working hours per week. So for a 3 days nanny, they may be working 30 hours a week, so 5.6 x 30 = 168 hours.
Payroll agencies are worth using, especially for the first year. Leaves you to concentrate on making the arrangement work, without needing to worry too much about the paperwork (online reporting) for HMRC. You can do it yourself via HMRC Online, bit of a learning curve but relatively easy once you understand how PAYE works.