I recommend that if you wish to become a stamp trader, dealer, or investor, you should spend years learning about the hobby before gradually and cautiously entering it as a business.
Minimally, you should join a local stamp club or two ( see the American Philatelic Society's list ) and also consider joining the A.P.S., which currently has about 40,000 members. Benefits of joining include a subscription to their monthly magazine THE AMERICAN PHILATELIST, and access to many useful services. The A.P.S. has many members from outside of the U.S. as well, so it is not necessary to be an American citizen in order to benefit from membership. Joining a local club will connect you to the local community of stamp collectors, some of whom will have had many years of experience and be excellent sources of good advice. You should attend local stamp shows every once in a while and begin to patronize local stamp dealers, and endeavor to find at least one that you likes and trusts. You should subscribe to LINN'S STAMP NEWS, a weekly stamp newspaper and the largest in the U.S., or one of the the major philatelic publications in your country, if you are outside of the U.S.. Both THE AMERICAN PHILATELIST and LINN'S are full of informative articles and ads - invaluable sources of information.
Beginning stamp collectors tend to be generalists who often try to "collect everything," forming worldwide collections or entire country collections. As they become more advanced, they tend to narrow their focus, specializing in particular collecting areas or issues. Ultimately, both general and specialized philatelic knowledge comes into play when engaging in the stamp trade, because then the primary motivation is to maximize profit.
Both Philately and the stamp market are analogous to cities with which a traveler must become familiar in order to find his way around. There are both general guides, such as the understanding of condition grading, and many particular bits of knowledge which apply to specific aspects, such as knowing which stamps sell for multiples of catalogue value and which sell for a tiny fraction of it, and knowing how to avoid getting one's pocket picked. Having to spend the necessary time and effort learning about the hobby before engaging in it as a business may sound boring and arduous, but it's preferable to the more "exciting" experience of trying to survive as a "babe in the woods."