(This is assuming you already have a group).
If you are to be the gamemaster, sit down and talk with your players. Shadowrun is a rich setting, and there's a busload of ways to play it (or most rpgs), and players may have very different expectations from the game. Some may want to emphasize on combat, tweaking and twinking the rules to build the toughest fighter, mowing down hordes of enemies with twin submachine-guns, others may want to solve tricky puzzles, while somene else may want to focus on social interaction, or roleplay a dark, gritty character in a dark, gritty world... then there's the people who want a sneaky, stealthy
Mission Impossible type of game. Or a magic-heavy, "Tolkien-with-Guns"- game. As a GM, you should also figure out what type/style of play would fit
you most, and (that's usually the hard part) try to find a way to make everyone enjoy the game... it's tough, but at the same time, it can be tons of fun.
If you are "only" in as a player, then first, gratulations, your gaming career has just become a lot easier
But most of the stuff above still applies. Try to figure out what kind of character you would like to play, and also what kind of
game you expect from playing Shadowrun, and talk about this with your GM. That can go a long way to making everything easier and more fun for everyone involved.
...
As has been said, look around, there's tons of useful (also useless, but still entertaining
) information here on Dumpshock.
Have fun, enjoy the game!