If you have never bought lingerie for yourself: start here. There is no fastening to work out, no cup to fit and nothing that has to sit in exactly the right place. It is the most forgiving thing in the category and the easiest to actually wear.
On sizing: go by the bust and let the rest hang. The straps and the bust are the only parts doing any fitting. Everything below is supposed to be loose, so a measurement that is slightly off disappears into the drape.
If you want to wear it out: satin, under a blazer or a cardigan. Satin reads as a slip dress the moment there is a layer over it. Sheer mesh and lace do not, however you style them.
If you want to sleep in it: check the straps before anything else. Thin spaghetti straps dig and slide off overnight. A wider strap or an adjustable one is the difference between a piece you wear once and one you reach for.
On care: hand wash cold, hang to dry. Satin marks and lace pulls, and both go out of shape in a machine. See all lingerie for the rest of the range.