Generally most places make S/M/L their normal sizes - and anything above their plus sizes. But different shops vary so it's hard to say.
Just look around - some things you might be better buying in plus size, others not so much. Don't feel like you need to put yourself into one section. But just - get used to a site or brand's sizing - which should give you a specific idea for that store seen as there's so much variance.
It depends on the store you're shopping at & really, you just have to "know" the store beforehand before purchasing online. For instance, if you normally purchase dresses from Anthropolgie, then you'd have an idea of where they draw the line on sizes like petite, plus sized etc.
Normally, anything above a 12 or sometimes 14(bottoms) which is normally an L, Large, is considered plus sized
Shirts...I've yet to figure out sizes there. All I know is I wear a Medium