How do you start trading stocks? and how old do you have to be?

[A Nobody]

In North America you must be 18 to open a brokerage account but you can have a parent open a custodian for you using your social security number and when you turn 18 the assets in the account can be moved to an account in your name

Before you spend $0.01 on any investment, you must know what you’re doing, why you’re doing it and how to do it. Before you invest in any security, the first investment you should make is in yourself, and the best investment you can make is by educating yourself.

The fact that you're still a minor should not prevent you from starting to study the markets and the produts traded in those markets. Begin your education by learning why you should invest and the importance of being able to make your own decisions or how the pro’s make theirs. Start your education by reading “Investing or Dummies” by Eric Tyson.
To continue your education select some of the following
Beating the Street by Peter Lynch
Bulls Make Money, Bears Make Money, Pigs Get Slaughtered, by Gallea
From Riches to Rags, by I.C. Freeley
How to Make Money in Stocks” by William O’Neil
24 Essential Lessons for Investment Success by William O’Neil
The Intelligent Investor, by Benjamin Graham
Common Stocks, Uncommon Profits, by Philip A. Fisher
One Up on Wall Street by Peter Lynch
Stocks for the Long Run, by Jeremy Siegel
Uncover the Secret Hiding Places of Stock Market Profits by Joel Greenblatt.
What Works on Wall Street by James O'Shaunessey
You Can Be a Stock Market Genius by Joel Greenblatt
Your Money and Your Brain by Jason Zweig

Websites that can provide instructions and help with procedures and terminology are Investopedia - http://www.1source4stocks.com/info/stock...pinion.asp or Smart Money
http://www.smartmoney.com/
Visit some of the more professional websites like Zacks Research - http://www.investors.com/default.htm?fromad=1
Some of these web sites will have advertisers who are worth looking into also. And remember, if they offer free information, get it.

And when you think you want to invest/trade, try some paper trading to test your skills without spending you money http://www.moneyworks4me.com/
and/or http://www.tradingsimulation.com/

You at least have made the right decision to start investing, this is the first big step and it won’t be your last. Keep taking those steps forward and along the way never take the advice from people that are not in the market or try to tell you not to invest.

Good luck on your journey, study hard and you’ll invest well.