Mmmmmmmm, pastis……
Water and ice cubes. You are not supposed to put ice in it until water is, but I have never heard a reasponable explanation for this.
You can also put mint syrup along with the water (comes in can-type bottle in french supermarket) and call it 'un perroquet' (parrot), or grenadine makes 'une tomate' (tomato). Or almond syrup makes it something whose neame escapes me. These are somewhat frowned upon by some serious pastis drinkers (think bar ends on risers) although still very nice to drink.
Finally if you are a rascally student type, pernod and snakebite and black is pretty devistating. Oh and pernod is by no means the best pastis, just the only one in most pubs in the yookay. The others are very noticeably different and worth a punt.
Santé! 😀