In terms of timing, I think it seems on where you live and what the demand is like. I am in London, and for both my children I had to confirm a place and pay a deposit while I was pregnant! So more than a year in advance. The norm may be different where you live but I am sure you won’t seem over eager.
Regarding choosing a nursery: I think that you will definitely get a feeling about the right place and if is important to visit a few places so you know it when you feel it! But overall you should think about:
The staff: Are they well qualified? Do they seem professional? Your child may well be here until they start school - you want to be sure that they will get a proper early years education. Most importantly, do they seem to genuinely care about the children? Do they seem happy to be at work?
The facilities: these don’t have to be fancy, but are they clean? Do they have enough resources (toys / books / different areas for playing and sleeping)
Childcare philosophy: sounds a bit wanky, but what I mean is do they share your views on raising children? E.g. if you like structure and routine, you probably won’t want somewhere that is very loose and free flow, and vice versa.
And some practical things:
The opening hours: Some nurseries have quite limited hours - will this work with your (and any other parent / caters) schedule?
The location: What will the drop off and pick up be like? Will am awkward route / one way system / missed train completely mess up your commute? You will be doing this journey every day for about three years so make sure it is doable!