If you want to make yummy "girly" drinks, sweet stuff like peach schnapps, blue curacao (which is mainly fun because it's blue!), etc. are fun.
The basics are vodka, rum, gin, tequila, and whiskey (although if you don't invite many salty old men to your parties, you can probably do wihtout). Irish Cream is fun, as is coffee liquor.
If you're on a budget, keep in mind that for a lot of liquor, there really isn't any perceptable difference, especially with mixed drinks. If you're sipping scotch, you'll easily notice the difference between various single malts and blends; if you're shooting ice-cold vodka, you'll never notice the difference between Gray Goose ($40) and Smirnoff ($12). So buy cheap (but not quite bottom-of-the-barrel). Tia Maria is probably 1/3 the price of whatever starbucks is putting out, but it makes a fine black Russian. Baileys I'm not so sure about. I remember not liking one cheap Irish cream I tried, so that might be one to splurge on.
Also generic cans of tonic water, ginger ale, cola, soda water, and canned juices are a good way to gear up towards a full-service bar. Without the little cans, it's too inconvenient to have all kinds of juices available and you'll usually have to settle for one or two.
