id go to the companys site to find the symbol. dont forget though that most linux distros are not companys, but communities. so you will only find commercial ones like mandrakesoft (now mandriva), and others like redhat.
you buy shares through your bank, providing you are over the age of 18 and have a current account with them. they will charge you per transaction in most cases.
http://www.reuters.com/ is a good site for market things, including risk level of investment. and if your age coresponds with your profile your stock will have to be in your parents name, unless amercian law is different.