Tamtam, I had this problem.
Molto is right. First thing to do is figure out where you go in a week, and what you need for it. Then start with basics.
Someone recommended a site on here about building a capsule wardrobe, and I took a look at that. I can't remember the address but you started with a pair of casual trousers in black, navy or biscuit and went on from there.
Unfortunately, about a year later, I then started to lose the remaining weight, so it all got a bit baggy, but one of the most important things I learnt from the experience was to get your shoes right.
For me, that was leather, softino-type plimsoles that I could run around in, walk a long way in, wear to soft play etc, but also looked smarter than a pair of trainers.
I built my casual wardrobe from that point with comfortable, fairly hard-wearing, cotton trousers in navy, black etc and then bought cotton t-shirts and tops to go with them, so I had an interchangeable set of garments to throw on every day without much thought, and still look put together.
Then, from that, I started to get more formal. I bought dresses that could be dressed up or down, so I could wear them with said plimsoles or sandals during a nice day, or dress them up with tights and courts for meetings or evenings.
I found that post baby, I needed different shapes. I too was very fitted pre-baby, but those silhouettes just looked weird afterwards. Instead, I went for dresses that had a slight tulip shape on the skirt and a wider sleeve.
Got to be honest, with money being tight, I dissected the M&S website and got most of my trousers and t-shirts from there because I knew they would wash reasonably well. My dresses I picked up from all over, and quite a few were cheap bargains I found in online sales (one of my best finds was only £20).