I've been wanting to do a cover of Wonderful Tonight . But I wanted to improvise the solo. I know the song is in the key of G but what key do I use to solo? Do I use G itself or E minor? And then what scale do I use, minor pentatonic? And the beginning lick , what scale is that in?
E minor is the same thing as G major so if you're more familiar with the minor scale use that. Also keep high up on the neck if you're playing an electric guitar cause anything below 10th fret just doesn't sound right whenever I play along with it.

The lick is also in e minor.
Good luck with improvising.

But for my money, the solo to Wonderful Tonight is one of those iconic ones that non-guitarists recognise, like Kossoff's solo in Alright Now. If you play a different solo to these sort of songs, people might assume that you couldn't play the origional solo.

We, as players, will recognise the skill you are putting of course, and will know you really can play, but Joe Public might not. Just a thought.
Yep, E Minor. And like Deano said, it is an iconic solo that people might assume you can't play. Try to play the solo close to the original if you're playing a show or for people.
you're a stone fox