It’s all about fit.
Your jeans should fit PERFECTLY. The hems should just graze the floor in whatever shoes you’re wearing, which is easy if you wear flats every day. If they aren’t the right length when you buy them, have them hemmed. It’s a cheap alteration that will make them look 100% better. Wide leg jeans that look like trousers are nice for work, and a little dressier, but if you can’t find any you like, stick to straight leg or bootcut. Also, no super low rise, no buttcrack showing, and no gapping at the waist. That’s another easy fix for a tailor so get it done. And your jeans should be a dark, uniform wash. Work is not the place for fading or whiskering or weird treatments (or, obviously, holes). Honestly, jeans that fit make you look older like, instantly.
For shirts, try nicer fabrics. Even good polyester (not the cheap crap from the juniors section, but the nicer stuff that almost looks like silk) is better than thick rough cotton polos. I honestly don’t like polos on women if you have an option- they’re just not usually all that flattering, although I have seen a few done well. But try wearing blouses instead with feminine details- cool patterns, pretty bows, ruffles, etc. I love blouses from the Limited. Tuck your shirts in, and invest in some pretty, thin belts. I love a thin belt with jeans and a nice blouse. Limited also has great adjustable belts, that can be cinched in at your natural waist with a cardigan, or can be worn lower on your hips in your belt loops, and they go on clearance for like six bucks.
And cardigans are a great alternative if you don’t love blazers- I have disporportionately large boobs, so blazers are hard and expensive for me. Cardigans are much cheaper and more versatile. I like them belted at the waist, but even open over a pretty patterned tank can work great.
Shoes should not be sneakers. Flats are key, and make sure they’re in good shape- no floppy soles or missing decorations.
As for where to shop- I love the Limited. I find their sizing very uniform, so I know I can walk in, buy a 2, and it’s going to fit. Period. And their cuts are pretty uniform, so it’s easy to spot flattering clothes even online. I like Target for shoes, especially flats, and DSW too. And for tops I mostly like off-price stores like Marshalls and TJMaxx. More digging, yes, but cheaper and just as nice as department stores. Someone else mentioned Express, which is great too.