Well I'm not quite that small/thin myself, so a regular fit men's shirt size S or a slimfit size M fit well on me. But as for tucking it in accenturating your curves: wear pants that sit on your hips, and make sure your pants are tight around the butt and that they're darker than the shirt. That's gonna help give the illusion you have a wider waist, narrower hips and smaller butt.
Another tip: many don't like the "muffin top" look to tucked in shirts (when its puffy around the waist, just above pants) but if you're fine with that look it does greatly help with hiding curves. But just make sure the "puffiness" goes over the narrow part of waist. You don't want to add width to what's already too wide.
If the shirts are very long, then I strongly suggest tucking them in or it will look like you're wearing a dress! Shorter shirts (around hip-length) can be worn untucked and look fine.
To help shirt stay tucked in better and they're long, before pulling up my pants I tuck the shirt "flaps" into my underwear into the leg holes from underneath. That helps keeping the front part of my shirts not get pulled up during the day. I've not used shirt stays though.
As for material, I prefer cotton all the time. Some of my shirts are cotton/polyester blends and that works for me too.
If you have money you could get the shirts tailored to fit you better. If not, maybe you could check out the boy's section at stores to find smaller shirts. If you'd shop online, many Asian brands have smaller sized men's clothing. I don't know any specific ones though, so you'd have to google it. But then a size S fits more like maybe an XXS in comparison, at least I noticed that with Asian brand underwear.
