start with a mock portfolio. yahoo finance among other sites lets you create dummy portfolios so you can test the waters.

don't trade excessively; or commissions, taxes, volatility and spread will kill your profits (or worsen your losses).

don't play with what you're not willing to lose as mentioned above.

make sure you keep good records so it's easier to report come tax time.

learn technical and financial analysis if you plan on trading. read buffett and siegel if you plan on investing.

if investing for the future, proper allocation is more important than choosing good stocks.

buy low, sell high. or buy high, sell higher.

it's good to learn as much as you can now since you're gonna have to deal with this for the rest of your life. my portfolio peaked at 150k a couple months ago, but the only stock i own got hit (lesson: don't put all your eggs in one basket) and i had to liquidate some for my down payment so i'm down to about 65k now.