Investing $25 per week for 20 years at an 8% return would yield $64,000. I don't know about you, but that sounds like pretty good spending money to have in my golden years, not to mention my 401K, Pension and/or Social Security.
I started investing $25 per month 3 years ago for my Godson and it is just at $1,000. His portfolio is up 27%, which means I only invested $787 and the rest is the stock market at work. Some months I did not invest anything and the value of his portfolio has reached as much as $1,400. But the market goes up and the market goes down! The good news is that over the long run the Market is constantly on an uptrend (see chart below).

So how do you get started......go to one of the following websites and sign up for a free brokerage account:
Etrade.com
Sharebuilder.com
Charles Schwab.com
TDAmeritrade.com
Next, talk to your employer about having $25 transferred into your brokerage account each pay period. This should not be a problem if you already have direct deposit. If this doesn't work for you, then you can do the following:
1) Transfer money directly from your checking account into your brokerage account.
2) Send a check into your brokerage account.
3) If you have online banking, setup electronic disbursements to your brokerage account (this is my method of choice).
Once you have setup your account and began depositing money, you'll need to determine where to invest it. You should start by thinking of companies that you are familiar with based upon your work industry or where you spend the most of your disposable income. For instance, if you like to buy shoes then you might be interested in investing in companies like Sketchers (SKX) or Steve Madden (SHOO), or whichever sells your favorite brand. If you are not sure where to invest, market indices like the Dow Jones (DJX), S&P (SPX) or Nasdaq 100 (QQQQ) might be your next choice. These investments will help you to diversify your equity position across the many companies contained in the index, not to mention that over the long term they are constantly moving upwards. When investing in the aforementioned, all you'll need is the price of at least one share of stock along with the cost of the trade.
Another alternative investment is mutual funds. These instruments usually cover a range of stocks and can be purchased with a minimum of $250, $500, $1,000 or more. I do not invest in them because they are covered by my 401K plan. But of course, the choice is yours in terms of where you want to put your hard earned dollars. The most important thing is that you put money away and that you feel comfortable with where it's invested.
You can learn all you'll need from your brokerage house via the web. For more insightful commentary on investing and possibly direction on which stocks to invest in, go to Motley Fool at Fool.com