Start with the links LouLou provided. Figure out your body type. Then go through your existing closet and pull out items that fit and flatter and are worth keeping.
Make a list of things you think you are missing. Start with seasonal items, so you don't get too overwhelmed.
Next, go shopping - but with a purpose!!! Wear one of the pairs of jeans, perhaps with a top and/or shoes you already own that fit and flatter, but just need a layer like a cardi or jacket, and some accessories to pull it altogether.
When you shop, buy outfits instead of individual items. This is a "shop with a purpose" strategy that works particularly well when you are building from scratch. Let's say you see an awesome blazer or cardi that fits great. Don't buy it unless you also pick out a blouse that will work under it. And maybe even a necklace, if you don't already own one you think will work.
I study mannequins and displays in the store for inspiration, but I don't stop there. I'll spend a long time in the dressing room mixing and matching and experimenting. Initially, I would not buy a top without a cardi or blazer and a cami and a necklace to work with it. This helps you build up a truly workable wardrobe fast, eliminating wardrobe orphans. Don't buy anything that you "figure you can find a cardi to match" later.
If you are wearing the jeans already, then you will be able to see in the fitting room if they truly work with the tops/cardis/etc. Always save receipts and don't take off the tags until you are sure you want to keep items. When you get home, try your new purchases on with other items in your
closet.
I don't know where you are in the process, but sometimes it can help to set rules like "don't buy anything that won't work with at least three other items already in your closet." This tends to work better when you already have some good pieces.
For spring if you are in a cooler climate, a great coat is an excellent place to start too. If you spend a lot of time outdoors, coats can cover up less flattering clothing and pull everything together.