Ask about the day's oysters, compare a small selection and decide how much lemon or shallot dressing you want with them.
Ask what has arrived
An oyster list can be a collection of unfamiliar place names. Ask the person serving which varieties they have today and how they differ. Starting with two or three gives you a manageable comparison.
Order a small selection and ask which oyster is which when the plate arrives. Keep track of the ones you enjoy by leaving the shells in place. It saves trying to remember an unfamiliar name at the end.
Try the first one simply
Restaurants often serve lemon and a sharp shallot dressing alongside. Taste the first oyster with little or none of either if you want to notice its own character. Then decide what the next one needs.
Notice the texture as well as the saltiness. Some oysters feel firm, others softer and fuller. Tell the server which you liked; that is more helpful for a second order than trying to produce an elaborate tasting note.
Let the occasion stay small
At a counter, a few oysters can be the whole visit. Ask for water and somewhere to put the empty shells, then take your time with the plate.
If you prefer them cooked, ask about grilled or baked oysters. A warm preparation gives a different texture and makes room for butter, breadcrumbs or herbs.
