I am not suggesting you to invest in high value rare and antique stamps in the beginning, but in the new issues at face value, and used stamps of recent period available in market. Your biggest benefit is that you have not to invest a big amount.
You should concentrate on collecting thematic stamps. A collection of stamps on a specific theme can give you better returns in future. The themes may be anything you like, i.e. 2nd World War, Presidents of USA, Space, History, Einstein, Birds etc. Stamps on a specific theme or famous persons become rare in short time, and can get high value when you want to sell.
You can collect stamps of specific country or specific period.
If you see any defective stamp anywhere, and getting it at face value or for affordable price you should buy it immediately, as defective stamps have more resell value. Here, defective stamp means a stamp having color shift, printing mistakes etc, and not physical defects like fold etc.
In addition to stamps, you can collect First Day Covers and other postal stationary (Post Cards, Inland Letters, Envelopes, Aerogrammes etc.).
Stamps and related items you buy should be in good condition. While buying used stamps, always choose stamps having smaller cancellation mark.