I've done this recently, and my advice would be to start by having a massive clearout. I used a company called Anglia Doorstep Collections. You book a date when they are in your area, and they collect saleable stuff from the door (you can be out/at work if you can leave the boxes or bags in a visible place). They do each postcode roughly once a month, so you don't have to do it all at once. At least some of the proceeds go to charity, and they take a lot of items. Be as ruthless as you can - I was amazed at what I got rid of - to them, charity shops and the tip.
That makes such a difference, as you can see what you need by way of furniture etc and potentially get rid of some of that, too, so you can rearrange your space.
Then get a decorator to come and quote. If you want everything doing, ask about the cheapest and most sensible way to do it, (eg if you start with the stairs and then have people delivering wardrobes or bringing ladders upstairs to decorate bedrooms you might find the paint gets chipped). Think about getting all the bedrooms painted in the same colours/the same carpet/flooring. That way you can save on costs and you'll only need to store one tin for touch up purposes. From there you can decide what you can afford to do in one go, and take it from there. It might be cheaper to get (say) all the bedrooms done together, if the decorators can do all the glosswork in one day, then come back for the walls, rather than do a room at a time their daily rate will be lower. Same applies to carpet fitters, or floor layers. They will advise on that though. If you can DIY, you can still use their advice.
Re shower - again, get quotes and ideas. Look on your local FB page for recommendations, and talk to people. Expect it to cost double what you were expecting, unfortunately.
Enjoy it. Once you get off the starting blocks it can be fun. I was surprised at how much I enjoyed choosing colours and so on, after years of watching things get progressively tattier. I really like living in my house now.