If i`m 13 can i get involved in online stocks? if no, why not?

Yes, you can. It is called the Uniform Transfer to Minors Act, I believe (although it could be Uniform Gifts to Minors Act) - it has been years since I received an account statement that said that on it.
I started investing on my own when I was 15, and had mutual funds for a few years before that. I pulled all of my money out of mutual funds, and opened an account at Scottrade. They offer the best commissions (the amount you have to pay to buy a stock, which is $7. They also charge you $7 to sell it, too) Many other online brokers will charge you $10, or even $100! Scottrade definitely has the lowest price, and I have been using them for years. I am VERY happy with the account service they provide. Besides, they are conveniently located, and they keep opening up new branch locations every week. It gives me the opportunity to walk in there and deposit a check, ask a question, or anything!
I would STRONGLY recommend that you begin investing at a very young age. I started at 15, which was a little late for me. I wanted to start earlier, but it didn't work out. I have learned SOOOOOO much by watching the markets, and investing in stocks. Stuff that class rooms can't really teach all that well. In high school (12th grade), I was basically the only one in business class with knowledge about the stock market (except for the teacher) And I learned everything I knew from constantly reading articles on Yahoo! Finance.
I am kind of getting sidetracked here, but I am just so excited to see young kids get a jump start on their financial life. Very few can even describe 'stock.' Learn from my mistakes: DO NOT invest in 'speculative' companies (speculative = gambling) Begin by investing in 'Blue Chips' (the biggest, oldest companies around, such as Coca-cola, 3M, McDonalds, Johnson & Johnson) The blue chips are traditionally solid companies, and they pay dividends (they PAY you to invest in their company! Every 3 months, Verizon pays me 40 cents per share that I own! It is kind of like free money, in a way) If you speculate in stocks, you will LOSE MONEY in the long run. I still own a few stocks that lost 100%, and I bought them when I was 15 & 16. One was a company called 'OmnySky' and they provided wireless internet service to Palm hand-held computers. I thought 'How could that NOT be the next big thing?' So I bought a few hundred shares for $500 or so. Then it went down. I had confidence in the company, so I bought more. It went down. I still thought it was going to be the next big thing, and bought even more when it went down some more. Then phone companies started selling the same service that OmnySky did, and I lost everything! ($2,000!) GONE, no way to get it back. That was speculation, because they were an 'up-start', and they didn't have a working business model.
But to open an account (I would *strongly* recommend ScotTrade) Go to www.scottrade.com and enter your zip code to see if there is a branch around you. Then go into their office, and ask for an "Investment Kit." Prepare beforehand to ask them any extra questions you may have about Scottrade or the stock market. They are very smart people. BUT they do not give you advise on what stocks to buy and sell.
One thing that may be a down-side: You have to deposit $500 to open an account. If that is too much for you, ask them if you can deposit $500, then have a check for $200 or $300 mailed to you. That way you will only have $200 or $300 in your account, instead of $500.
If you need money to get to $500, ask your parents to loan money to you to get to $500. Make sure they understand you can withdraw the money to pay them back, that you WILL NOT be investing any of their money. If your parents will not loan you the money, ask them if they know of a relative that will. Perhaps you have a grandpa or uncle that invests in the stock market, and they may be more than happy to help you get started.

But the main thing that you need: A parent or guardian to open a "Joint Account" with you. Until you are 18, you can not have your own account.

The first time I went into the ScotTrade office, I was really scared. I thought I was really out of place, and I wanted to get out of there as fast as possible. I brought along the nicest pen I could find, and when I needed to sign a form, I pulled out my really nice pen, and instantly felt embarassed. The stockbroker behind the desk was using a Bic pen! It wasn't until many months later that I realized they are normal guys that do normal things, and they even talk about normal things! Since I was so young, I had a total mis-understanding about stockbrokers.
But be careful, any brokerage you go to will have people that don't know everything. Every once in a while I will be walking through an office building downtown, and I will go by a brokerage. Once I went in and asked what I had to do to become a stockbroker. As soon as I walked in, the person behind the desk gave me a really mean look and didn't say anything. I looked on her desk, and she was reading a magazine! She was mad at me for interrupting her magazine time! Then she said I would have to go to a University to ask them what I would have to do to become a stockbroker! That was ridiculous! All she had to do was smile and say, 'you can ask one of our stockbrokers that is sitting right over there,' but she didn't. That left me with a really bad taste in my mouth, and I thought everybody in that office was like her. I was sure wrong! Also, brokerages will sometimes hire students, so the students can get out of the classroom and gain some experience in the real world (it is called an 'internship.') I once dealt with an intern when I walked into ScotTrade, and he didn't really know anything. So make sure you talk to somebody that knows their stuff.

If you have any more questions about the stock market, getting an account set up, ANYTHING, just click on my name to get into my profile, then send me an email, and I will be glad to tell you more, or help you out in whatever way possible.

(this turned out to be pretty long, so hopefully you ended up reading it all!)