You could look using internet sites like Childcare.co.uk (click link, then modify the postcode to fit with your actual location).
Netmums (local childcare board)
Gumtree Cardiff
NannyJob
You could use an agency to help. Not sure which agencies cover Cardiff. Tinies (Bristol) might cover that area. Nannies On The Net may cover Cardiff.
Salary wise... up to you. I would guess at somewhere between £7 and £10 gross per hour, depending on how experienced you wanted someone to be. Calculate how much you can afford, then offer below that, so you have some ability to raise salary after end of probation, end of 1st year etc.
How soon... no harm in my view looking 6 months or so ahead. If a nanny is in employment at the moment, they may know their job is coming to an end say around September (due to children being older and going to school full time, for example), thus that nanny may start looking for their next job in May/June, maybe even earlier.
Have you got your head around some of the legal things... like being an employer and what that means? Have you looked at PAYE, how that works, decided if you want to do that yourself or pay a payroll company to help you with it.
Would you be wanting to part-pay using Childcare Vouchers? If so, then I think in Wales you need information about childcare at home approval scheme.