There is no “best” on-line company
Mostly all brokerage firms provide their clients with on-line services, including trading platforms, latest market & financial news and research.
Customer need to select the site that is best for them. Traders have requirements in a site, while investors have other requirements.
Although most sites are geared to general securities and commodities, however customers may have special needs for the types of products they trade, and the markets in which they trade.
It seems that the most popular firms for on-line investing all of which provide excellent platforms and services are; Scottrade; Chas.Schwab; TDAmeritrade; Fidelity; E-Trade and Thinkorswim.
Based on your question, the last thing you should be looking for is an "on-line" site,
You should first educate yourself about the securities markets and the products traded on each.
Get to learn the terminology, the rules and the various strategies.
visits to some websites like MSN Money and Yahoo Finance. (http://moneycentral.msn.com/home.asp http://finance.yahoo.com/ )
While at MSN following the strategy lab analysts to get a feel for what the pros are doing and why. This site has some basic information for beginners. If any site offers free information, take it. Other website that can provide instructions and help with procedures and terminology are
Investopedia - http://www.investopedia.com/ Stock Charts - http://stockcharts.com/
Other - http://www.investorshub.com/ http://www.1source4stocks.com/