Hmmmm...

I think it's difficult to come up with a prescribed list of must-haves - people are too individual and tastes change over time
The only thing I can think of that would be broadly applicable would be to start saving now for a great watch that you will wear forever - something along the Cartier, Tag or Rolex SS line. A significant purchase but one that gets great cost-per-wear. Check out authorised secondhand dealers or vintage watch traders if you can - watches like this last well and there is a strong second-hand market - people do frequently sell watches without ever having worn them

But it can be a great deal for you if you're in your early 20s and looking to start your collection.
What I would try to avoid in this age-group is buying pieces that are what I call 'interim' jewellery. People can end up with multiple pairs of diamond studs this way and the smaller ones rarely get worn again. You can pass them on to a daughter or a sister but you would be better off, IMO, waiting and saving for the ones you really really want and will wear forever. I don't own diamond studs yet for this reason - I want a pair of Asscher diamonds around the 3cttw mark. I'm happy to wait. I have a pair of simulants from carat.co to keep me happy in the meantime.
Otherwise, just try as many pieces on as you can, check out as many estate jewellery stores as you can and start working on a list for yourself of what pieces you would like to acquire - make it as fanciful and dreamy as you like. Then edit, cut down to realistic pieces and make a bit of a timeline. There are other financial goals to be set for this time in your life also, but a great jewellery collection doesn't happen by accident either. If it is important to you, you can direct funds to jewellery while perhaps not spending on things like clothes or bags so much...
Of course, not everything has to be planned you know - jewellery has a way of leaping out at us spontaneously...