I have never been paid for bartending, but I have hosted fairly large cocktails mixing session at work and thought dozens of people how to mix. Given that I found a way for my company to pick up the tab for one of my hobbies, I guess that still counts as a side line.
I started with two classes: the free intro at the San Francisco School of Bartending and the mixology class at Bourbon and Branch, also in SF. When you get down to it, there are only a handful of techniques, less than a dozen tools and about 20 ingredients that will cover 95% of the drinks you will ever make. The technical part of the job is rather simple and you can learn it pretty well from a book, but I find watching videos more helpful and this app is probably a very good start:
http://appcrawlr.com/ios/speakeasy-cocktails-learn-from-The biggest part of the job is to be an entertainer and that's all about being good at chit chat and being a bit dramatic with everything you do behind the bar. When you go to a restaurant or a bar, sit at the bar and watch the bartenders do their thing, ask them questions. This is the best way to learn.
You only need a few tools to get started, which will cost you about $100. You need your well liquors, a bunch of mixers, and plenty of fresh citrus fruits so add $150 on top of that and you are ready to host amazing parties. Most of this stuff is very shelf stable so you are buying for the long run. After blowing your friends minds for a few months in a row, I am very confident you are ready to serve strangers and it probably won't feel like work at all.