I don't think there's a right or wrong, really, although I wouldn't start buying in the first trimester.
You're 22 weeks! If you're not ready to take the plunge yet, that's absolutely fine. If you fancy it, start browsing online for things you'd like to look at. Do you know what sort of pram you'd like? Might you want a moses basket? What about more practical things like baby baths which you probably will need? Clothes for baby? Just do some 'window' shopping without the intent to but yet, just to see what's available and what sort of things you'd like as well as stuff you'll definitely need. You could also plan a budget too if you want. It could be very useful.
I'll be 26 weeks tomorrow and we have all the 'big stuff' from having our daughter two and a half years ago. We've got a bedside crib, baby bath, pram (not upgrading to a double) and so-on. We've bought bits for this baby. Some clothes (not loads), and a few practical things like nipple cream as I'm hoping to be able to breastfeed. We've got some nappies in a couple of different newborn sizes, but only because they were crazy cheap in Morrisons a couple of weeks ago! One thing we don't have and definitely want to get is a baby carrier (one which straps to the parent's body to carry baby in). I've only just starting looking into what's available and what might be the most suitable for us
There's no rush, but start getting some ideas together. Make a wish list on Amazon (even if you don't plan to but from Amazon) so when you do decide you're ready to start buying stuff, things are there ready and a lot of the research will have been done already.
It's perfectly reasonable to start picking bits up as and when you see them. You don't have to leave everything until the last minute then panic about getting everything you need when you would prefer to be relaxed and prepared.