Love this topic.
I've been looking around the internet for the past couple of years as I tend to be a hoarder when it comes to clothes and never really had a good idea of what I should be looking for when it came to a base wardrobe. I was always in that situation of having too many clothes but nothing to wear.
A few years ago a very lovely friend helped me to go though it all - as other posters have suggested, getting everything out, trying it all on and getting rid of what wasn't being worn. It also helped me to identify those wardrob 'orphans' (I can't remember exactly where that term comes from, but I love it). I then realised that I had never shopped with any purpose - Instead, I might have the vague idea that I should be buying clothing (before it all fell apart) and then would wait until I fell in love with pieces.. that often went with nothing else I owned.
In the end of that exercise I realised that the reason I was struggling was that I was only wearing a tiny portion of my wardrobe. The rest was either too old, too uncomfortable, too unflattering or just didn't go with anything else.
Since then I have tried to be much more systematic and picky with purchases. For any base items e.g. a pair of pants, the fit has to be really good and not just good enough. Same goes for jackets, cardigans or shoes. Colour too is important but I don't restrict myself as much as some people. Base Items are in any of the neutral colours (I do tend towards black/navy though). I've found that I don't like patterned bottoms (as much as some blogs recommend them). Instead, I like to have a range of patterned, block or otherwise bright T-Shirts or other light tops.
For casual my uniform tends to be:
Jeans/black pants with a patterned top (sleeveless), cardigan and often a scarf for extra colour. If it's cold, i can add a base top underneath and change the cardigan for a jacket.
For work/more formal:
Either keep the black pants or swap to a pencil skirt, same tops and cardigans work. Change the scarf to chunky jewellery.
For some reason, as much as I love dresses, I tend to always gravitate towards pants/jeans unless i'm getting really dressed up. They just don't feel 'casual' to me, no matter how casual they really are.
My advice is pretty much the same as what has already been said -
1) clear out that wardrobe. Figure out if you aren't wearing some things because you lack the item to go with them (some tops/jackets only go with low waist pants, others with high waist skirts etc.).
2) If it doesn't fit, get rid of it (or get it altered).
3) If you don't like it (even if you feel you should wear it because it's 'appropriate' or because of how much money you spent) get rid of it. Someone else out there could be making use of it. Free that space up.
4) figure out what basics you are missing and 'how' they should fit - just knowing you want a black jacket isn't enough - where do you want it to sit at the waist? Do you want fitted or loose?
5) find out if you like patterns or blocks. Do you like to wear them on the top or bottom or in accessories?
6) Realise that what you like on the hanger or on other people, isn't always right for you. (It took me one pair of patterned pants to realise I hate to wear patterns on the bottom half - much as I love them in magazines and on other people).
The last advice is to then practice it. Last year I was travelling for several months with limited baggage space which really made me look at my wardrobe again. I took 2 pairs of pants, one of shorts, two dresses (one formal, one maxi), six patterned tops, three neutral base tops (for under), two cardigans, one warm jacket, one light formal jacket, one pair of dress shoes, sandals and casual ballet flats, two scarves and of course, underwear.
Because I didn't have options, I made what I did have work. When a couple of my items became too worn out, I found I really knew what I wanted as replacements. By then I knew what sleeve length, neckline, fit, colours, fabrics and other details would be most suited to my needs and was far more particular when shopping.
So once you have a base 'capsule' wardrobe, hide everything else away for a month or two. Give it a real test. You will soon figure out what the gaps are and what isn't working.
In a way I think it's taken alot of the fun out of 'shopping'. Where I used to wander around aimlessly for something that caught my eye, I'm now on a mission with specific requirements ... but I'm saving money! and I know that I always have something to wear.