In addition to the good advice already given:
Consider bumping your suspension up a couple of notches, however little your pillion weighs.
I finally got my 60-year-old mum-in-law on the back for a blatt, as it was always something she wanted to try - She is tiny and probably weighs about the same as my dog, but even that affected the feel.
Be prepared to find yourself forced forward, even when just riding straight, as you're probably used to having loads of space and stretch room.
Be ready for the inevitable lid-bash and sudden extra weight pushing your forward under braking. Brake earlier, more gently and expect to need more force than usual.
You may find you need a bit more revs to pull away than you're used to, so expect to stall the bike a couple of times and be ready to stop it dropping. Allow more time when pulling into traffic.
The bike may feel wobbly on the steering for the first few times you pull away until you get used to it, especially if you move your own bodyweight around a lot when riding. Expect filtering to also be a bit more wobbly/heavy and don't try throwing it around in nimble moves until you're thoroughly used to it.
Also, let her try out different handholds, to see what is most comfortable for each of you.
My Mrs holds the lower front of my jacket, as it feels secure to her but leaves my upper body to lean around. Topboxes with backrests are good.
Another thing mentioned - At higher speeds, some bikes develop a vortex between rider and passenger. Bloody annoying, as you'll find your head gets battered and wobbled around.
If you want to show off the acceleration of anything, make damn sure you're clear and safe to do so.
But generally just take it easy for a while.
More than anything, you'll want her to trust you. She is putting her life in your hands, so gaining her confidence in your abilities as a safe rider is paramount.
Make sure she knows this is your fist time carrying a pillion and is aware of the various little oddities that may occur, just so she appreciates the little learning curve you're about to go through.
Once she is happy with you, THEN you can start working on the fun stuff!!