| | Re: placement of "now"
The problem isn't just "now" (which is flexible in its positioning) - it's the choice of verb forms, which are sometimes not possible, and the meaning. Here's a rewrite of the sentences which clarifies them :
1. I am not going to cook lunch now/yet. I want to read a novel instead.
2. I don't know now what my uncle is planning for my birthday. This means : I knew a few days ago but he's changed the plans and hasn't told me the new ones. Do you mean : I don't know what my uncle is planning for my birthday yet meaning : Up to now he hasn't told me anything.
3.I don't want to do what you asked right now, but I'll do it later