I was in your position a few years ago and I found it useful to first think what I wanted out of the arrangement, and what were my priorities. When I just wrote down all the little and big things that were important to me, it really helped me interview nannies - I knew what I was looking for
I realised I wanted someone who was:
Friendly, professional, intelligent, able to come up with lots of suggestions about things to do / places to go, etc, someone who could get on and plan the kids days and activities by themselves. Someone with a 'can do' attitude.
I realised I cared less about:
strict routines (although I did care about nap times).
Cleaning / other duties
I realised I didn't want to:
micro-manage!
I wanted someone who would do:
lots of fun and active things with my DC,
take them to toddler groups, library, etc
cook a good, healthy main meal at lunchtime (so I could give them a 'tea' rather than a 'dinner')
do lots of crafty things (painting, model-making, messy play - things I knew I should be doing, but didn't get round to very often)
leave the kitchen and playroom clean / tidy at the end of the day.
Help out with ironing DH's shirts while the kids napped
I realised I didn't want someone to:
Clean the DC's bedrooms
Do the DC's laundry (but put the clothes to dry if they noticed them sitting in the washing machine!)
Clean the house.
When you've worked out exactly what YOU want, the interview questions are less difficult.
Oh, and one last thing. I felt very nervous about the whole 'employer' thing - tax, payslips, drawing up contracts, etc. Don't worry - there are lots of companies who do it all for you for a small fee each year. It's much easier than you'd ever imagine.
Good luck!