When I lived by myself, I rarely entertained. I kept one dusty bottle of vodka on hand, in case guests dropped by unexpectedly, but that was it. I was intimidated by the process of having a fully stocked bar since I didn't know exactly what I needed, but I also loved the idea of having friends over for cocktails. When I first met my husband, his place felt like an old library (in the best way!) - he had dark leather furniture, an entire wall of books and, of course, all the tools to make a great cocktail. The first time he made me a martini I felt so grown-up and though our liquor collection has admittedly gotten a little out of control, here's a short guide to creating a nice starter bar.
Alcohol - Four main spirits to stock: whiskey, gin, tequila, and vodka. They provide the foundation for many drinks, but are also perfect on their own. Vermouth (both sweet and dry) is also used quite often. While I typically prefer rye whiskey, bourbon is also an excellent choice for your entry point into "brown" liquors. As for gin, there are quite a few new options on the market, but it's hard to go wrong with a classic English dry variety, like Plymouth. For most tequila, I go with a "blanco" version since they aren't aged as long as other tequilas and are more neutral flavored. Simply be sure you always buy 100% agave and not a "mixto" version. I'm not a big fan of vodka (I find it completely tasteless), but it's one of the most popular spirits and good to have on hand.
Additions - Club soda, tonic water, and ginger beer/ale are common mixers, whether it's to add some effervescence or round out the flavor of the drink. Many cocktails also call for a sweetener (i.e. simple syrup, honey, agave, etc.) and while you can easily make your own simple syrup (it's equal parts sugar/water boiled down), I always have a bottle in the fridge. Bitters are the final key component when stocking your new bar. They come in a wide variety of flavors and provide a wonderful complexity and body to many cocktails, but when starting out, all you need is a bottle of Angostura bitters. They are probably the most famous variety and are an essential ingredient in classic drinks like a Manhattan and an Old Fashioned.
Tools - A cocktail shaker, strainer, and bar spoon are three items you need to mix drinks. For measuring, I prefer a clear small measuring cup that offers multiple volumes and conversions (i.e. teaspoons, ounces, etc.), rather than jiggers. As for glasses, my go-to are: cocktail/champagne coupes (if you're only going to get one type of glass, this is the one to own), a lowball tumbler, and a collins glass. I also make sure to keep straws on hand and cocktail picks for skewering garnishes.
Extras - Many cocktail recipes have some kind of garnish, so I like to keep a variety of citrus, olives, and cherries on hand. While Luxardo maraschino cherries are an investment, they're vastly superior to the neon-red version at the local market.
Ice - While it might seem like all ice is the same, using larger cubes allows cocktails to cool quickly without diluting the alcohol (plus they look cute).
Building a bar doesn't have to happen overnight - and in fact, alcohol lasts almost indefinitely, so take your time and enjoy the process. Resources: Lucite tray - CB2 Alcohol - We buy most of our alcohol, mixers, and extras at K&L Wines. Ice cube trays - large and small Champagne coupes - Anthropologie Lowball tumbler - Vintage Collins glass - Crate and Barrel Straws - Amazon