Focus on the object or nimitta? Once I reach a nimitta during jhana meditation, do I then move focus to it or keep in the object? Do I ignore the nimitta, or is it one of those things that is very hard to ignore?
I presume that your object is the breath, crossing the anapana spot (under the nostrils, on the upper lip).

You must be careful since wanting the Nimitta to appear will actually prevent it from appearing. As long as you desire such things, to experience Nimitta or Jhana, it will simply never happen.

Let go of such expectations, completely. Also, by wanting Nimitta to appear, there is a very high chance of it being the product of your imagination.

You must let it be, do not give it any attention. Nimitta will strengthen, stabilize, and merge with the object on its own. Always remain with the object.