This may sound weird, but... GarageBand. Find some good plugins/synths that are compatible (for example, look up a neat one called Automat), and start messing around.
Wow, completely ignored, ok...
Also, since some people think you need to spend money, and you say you have none, take a look at REAPER - its trial version has no limits and doesn't run out, so you can just purchase the $60 license when you're ready.
Another free alternative is
http://audiotool.com. It's one of the things I got started on, plus it has a great community built in. Oh, and did I mention completely free?