I'm not 30 but coming, hopefully, slowly. Building a closet is sometimes cost and we end up spending money in things we will never wear or that are one season trend and we will never wear again.
The Basics on this list are items that are the glue that helps the rest of the wardrobe together because it goes with everything and you can over and over again

Black blazer - if there is one item all closets without any exception should have is the black blazer. this piece is so versatile even if you want to wear your jeans and snickers everyday you can still wear your black blazer and look so elegant. Pair the blazer with jeans and high heels for a night out or with black trousers and shirt to a business formal.

Good pair of jeans - a woman struggle: to find the perfect jeans. You will buy a lot of jeans in your life and maybe you will only really loved one or two. The trends also dictate a lot in this area. We used to use the boot cut jeans and now the trend is skinny jeans. In my opinion go for what is more flattering to your body. Choose something in between of the boot cut and skinnier and you will have all season jeans.
Jeans can really adapt to different occasions.  You can wear with high heels and a blazer for a polished look of with some flats and white shirt for daily.

Black and nude pumps - if you can't get both get the black one. But if you can get the nude also. Both of this colors are perfect in shoes and you will not need anything else. Use the black ones to work and the nude to a date night.

Trench coat - woow burberry. This is my dream trench coat. They are perfection. I now live in UAE so I really don't need trench coats or coats of any kind. But if I move again to a country that has some kind of rain and cold. This is on top of my list.The trench coat for me in only one colour: camel.

White shirt - the all time classic. The white shirt is actually not easy to find, or better there is plenty of choice but you will buy a lot until you find one that suits your type of body.  But buy it spend extra time and if possible money because you will wear it for ever,

Classic jewelry set - Probably you will get some pearls from your mother or grandmother. if not get a classic pearl earrings and a match necklace. They are iconic pieces and you will look so classy.

Signature perfume - You should find your scent by the time you arrive the 3 0. This you should look for something that describes your personality. Weather you want something more sweet if you are kind, gentle and you like to be seen as someone who is always there for others. Or a strong scent to impose your person in a room or if you are a leader. If you are a free spirit maybe you will go for more fresh scents.
The important is that by 30s you have found the scent that defines you.
One thing I love about my husband is that whenever I go if I smell his perfume i start looking around to see where he is and when I travel I always take a little of his perfume to remind me of his presence.  A signature perfume is one of the most important things. And as Coco Chanel said "a woman who doesn't wear perfume has no future"