Yahoo Finance is user-friendly, so navigating it will be simple. First, go to their website at http://finance.yahoo.com/. You can also go to the Yahoo home page and click on their "finance" link. Once there, you have a number of options. The "investing" tab at the top will have a drop down menu. From there, you can choose the link that has the information you're looking for.

How can Yahoo Finance help me with my investments?
Yahoo Finance is a good tool not only for researching mutual funds, stocks, and other investments, but for learning about them altogether. Some of the features and pages Yahoo finance contains that will help you with your investing include:

Market overview
This page will give you an hour-by-hour view of the market and highlights top earners and gives insights into the various stock prices.

Stocks
The stock link on Yahoo Finance features a number of things that can help you with your investments, including:
Stock screener, which allows you to screen your own stocks. A stock screener allows you to find stocks by entering in certain criteria, then displaying all of the stocks that meet your specific criteria. The stock screener lets you to search thousands of potential stocks using a number of different criteria, including price-earning ratios, bargain growth, or large cap value. You can create your own stock screener screen, or you can use a number of preset screens. There are also screeners available for mutual funds and bonds.
Annual reports. You can find a company's annual reports for free on Yahoo Finance. This information is useful for your stock research, and companies can be searched alphabetically, by company name, or browsed by industry type.
Financial calendars. This stocks research link allows you to see a company's financial calendars. You can search by IPOs, stock splits, mergers, and other pertinent information.

Mutual Funds
In addition to the screener listed above, the mutual funds page on Yahoo Finance can help you with providing you such tools as:
Top Performers. Learn the top performing mutual funds and search them by the overall top performers. Or, you can select from the top-performing US Stock funds, Bond funds, international stock funds, or hybrid funds.
Fund prospectus. Yahoo Finance also provides a free fund prospectus for mutual funds that categorizes the funds alphabetically, by fund family, or by investment objective.

Education
If you need to learn more about various investments, articles about the basics of investing, frequently asked questions, and how to choose an investment will all help you to better understand your investments in general. Yahoo Finance has a number of these articles.

Overall, Yahoo Finance is an excellent free online tool that will help you with your investments.