Step 1: purchase Adobe Flash ($700 US, but adobe offers excellent student discounts on their software)
Step 2: learn Action Script 3 (AS2 is tempting for a lot of beginners but, trust me, AS3 is similar enough in structure and much more efficient in execution which is super important in flash games), google is your friend. But kirupa and tutorialized are two very good places to start. You may also hit a bookstore and pick up a copy of the AS3 bible or something similar, there are a lot of really good books.
Step 3: Do it. It is not an overnight task, if you can build a team, a programmer, artist, game designer, and audio designer, it will go a lot quicker.
I strongly recommend getting an official release of Flash, if you are serious about game design there will be a point you might want to enter into official competitions with your games (at indie game festivals for instance) you can't if you got your copy of flash from the torrents.