Three-dimensional (3D) topological insulators (TIs) are a new state of quantum matter with a bulk gap generated by the spin orbit interaction and odd number of relativistic Dirac fermions on the surface. The robust surface states of TIs can be the host for many striking quantum phenomena, such as an image magnetic monopole induced by an electric charge and Majorana fermions induced by the proximity effect from a superconductor. Recently, several classes of materials were theoretically predicted to be the simplest 3D TIs whose surface states consist of a single Dirac cone. By investigating the surface state of these materials with angle-resolved photoemission spectroscopy (ARPES), we demonstrate that the surface state consists of a single Dirac cone; and with appropriate hole doping, the Fermi level (EF) can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Furthermore, by simultaneously introducing magnetic and charge doping into Bi2Se3 to break the time reversal symmetry and tune the EF, we observed a gap formation at the Dirac point and successfully realized the insulating massive Dirac fermion state. Our results not only confirm that a single surface Dirac cone exist in 3D materials, but also pave the way for promising applications.