This certainly depends on your own desire to learn, if you have all the proper tools and mixes, your budget, and what your friends like to drink!
If your girlfriends love Cosmopolitans, perhaps buy some of the ready-made mixers. Some already contain vodka, so be sure to check out the label before you buy.
For more fun, be sure to use martini glasses!
If your friends are sports-watching, beer-drinking type of people, simply serve beer. Of course, find out which type of beer they like: out of the can, light beer, or craft beer. Beer drinkers can be quite particular, so be ready for lots of different answers.
An easy idea is make a simple punch. You can serve it in an old-fashioned punch bowl, or in a simple glass pitcher. Perhaps sangria, or spiked iced tea or lemonade.
Serving cocktails is about the “fun”, so feel free to experiment with some unique recipes, too!