Keeping a bar can be an expensive proposition, so choose judiciously. It's your bar, so if you like martinis, then keep gin and dry vermouth in the house.  Scotch and soda guy? Buy accordingly.  Remember the following:

For cocktails and straight shots, you want higher-quality liquor. To get more info on quality liquors, go to your local liquor store and ask for the best brands of the liquors you drink. Find out if they do tastings (most shops hold wine tastings and some have new liquors available for customers to taste ). Or you can ask your favorite bartender to try a few; and

Creating a cocktail specifically for the evening? Don't buy a bunch of specialty liquors you will only use that evening. For example if sweet liqueurs aren't your thing and a drink calls for Grand Marnier, you can probably make due with Triple Sec, an orange liqueur that is less expensive and more versatile. With all liqueurs, we recommend buying smaller bottles (500 ml or less), or getting the airplane/mini-bar sized bottles. They run two bucks and you're not stuck with a nearly-full bottle of Godiva liqueur after your "Chocolate Underground" party.

"Rail" Liquor:  any drink that uses juice or multiple mixers can be made with lower-end liquor.  Every bar has a "rail" where the base liquors are kept.  Nothing fancy, fine for making batches of drinks.  If you ask for a margarita, unless you say Patrón Silver, you get whatever tequila was on sale from the liquor distributor that week and usually are none the wiser. Is there a difference between rail and Patrón?  Sure, but who can tell through the Triple Sec, lime and salt. If the terms "Russian Quaalude" or "Blue Whale" are a part of your drink vocabulary, then it probably could use some updating. So, learn a few drinks and consider getting a cocktail book for parties and the like. Mr. Boston's is the bartender's bible, but there are many others available, as well as several websites, like  Drinks Mixer and Drink Nation. The cookbook you bought probably has a few drink recipes in it.  The key is experimenting and changing it to suit your taste. 

Our research (asking random people in addition to our own experiences) shows that these drinks might hold you for most evenings. Chances are, it's not the drinks your company's there for, but a Bombay Sapphire and tonic, some conversation, and next thing you know, it's 5 a.m., she's wearing your shirt, and you swear she said she had to get home some time around 10 p.m.  The recipes below are how we do it:  

Basic Mixed Drink Primer