A gin and tonic is traditionally served as a highball but it's commonplace to serve it in a lowball / old fashioned glass. Preferred servings can vary from a 2:1 ratio to a 4:1 ratio (tonic : gin) but I recommend starting right in the middle with 3 parts tonic to 1 part gin and you can tweak from there depending on your tastes. Personally, I prefer as much ice as I can fit in the glass too.If you want to keep it simple then go with a slice/wedge of citrus for garnishing (lemon, lime, orange, grapefruit, etc.) but if you want to really amplify the flavour profile of the gin you are pouring, try to match your garnish to the most prominent botanicals featured in the gin.For example, I am one of the co-founders and distillers at Threefold Distilling and we make an Aromatic Gin that features 11 botanicals but the most prominent are grapefruit, rosemary and lavender - therefore, we recommend the garnish be a slice/wedge of grapefruit and a sprig of rosemary.... by doing this, it highlights the key features in our gin.... but, in saying this if you are caught out and you don't have grapefruit on hand, any slice of citrus will still make an excellent gin and tonic.The tonic is also a very important aspect given that it makes up 30%-80% of your drink. The brands that I like using are CAPI (Australian) and Fevertree (UK).