Hello, Jacques, and welcome to the forum!
I can well believe you feeling worried. Any operation is a big decision and often a life changing one. The best bit about this is that it will be a change for the better! Focus on that.
There's no point in skirting round it, having a TKR is a very painful procedure but there are things you can do to help you cope.
First, immediately after the op you will have plenty of pain killers to keep you pain free. You will most likely have a spinal anaethetic so for the first few hours that will cover you.
Before you have your op make sure you discuss your post -op medication plan with the surgeon so you know to whom you should go for refills when the need arises. If he's one of these "suck it up and get on with it" types, then talk with your personal doctor and get him organised to give you prescriptions.
You will need something strong for the first few weeks, maybe even a cocktail of 2 or 3 different drugs, say oxycodone, diclofenac (Voltarol) and Tylenol at different times. In several of the threads there is a pain score chart that gives advice on how to time your medications in line with your pain level (and the doctors orders, of course!).
As for the nausea, I take it you mean during or immediately after the operation? Have you experienced post-op nausea before? Either way, mention it to the anaesthetist when he comes to see you. They are very used to people either feeling sick or being anxious about feeling sick and will readily give you something to hold it at bay. All of these things we are accustomed to so please don't worry.
It's a big step to place your complete trust in people you have never met before for a procedure you have no idea about what's going to happen. If you have any other questions, please, please post them and we'll do our best to answer and reassure you.