I have both perspectives on this as a run a cleaning business and used to have a cleaner myself. Sounds like having someone is the best possible thing you could do!
A good cleaner should be able to walk in and see immediately what needs doing, but equally you can set the tasks. It's important to be clear about what you would like done, for example bathrooms, bedrooms, kitchen and how often you would like them done.
The hours depend on the size of your house really. A 3or 4 bed house would take 2-3 hours as a rough guide. A good cleaner should be able to give you an estimate after seeing your house.
If you would like ironing done, simply ask. It comes under the remit of "housekeeping" as far as I'm concerned.
Ask around for someone as word of mouth is usually a good way to find someone trustworthy. Interview the person as you would for any job, see how you get on with each other.
It's a good idea to get things relatively tidy before your cleaner comes in - the less tidying she has to do, the more time available for cleaning. If, however, you need tidying up doing, just ask for it to be done as part of the job.
Don't clean for the cleaner! I've seen it done! (An absence of skanky underwear is always appreciated though).
Try to be out when the cleaner comes. It's difficult not to get involved otherwise. You'll end up with a more efficient job being done.
Be specific if you want them to use certain cleaning products and make sure you have a good supply. Some cleaners will use their own products and equipment.
Finally, pay good money. You are more likely to end up with a good, reliable and thorough cleaner if you are prepared to pay a bit more than normal. You know - monkeys, peanuts, etc...
Good luck!