Hello,
When you make the frequency high enough, almost every structure will radiate (except when fully enclosed by a good conductor).
Metallic structures will radiate easier when their size is no longer small compared to the wavelength belonging to the frequency of the source.
In most cases you want to have a predefined radiation pattern, predefined input impedance, radiation efficiency and useful bandwidth. In such cases antenna design becomes important.
To learn how to use the simulator, it is best to start with some simple standard antenna designs (halve wave dipole, rectangular patch antenna, etc). Check whether your simulation results match published data.
To learn how to shape a metallic structure so that it behaves as requested, requires you to study antenna design. In many cases a standard design can be modified to fit your requirements, but you need to know some basics about relation between current in a structure and generated far field. Many antenna structures behave like transmission line sections, so you need to know transmission line theory also.
In narrow band antennas the current distribution is mostly dictated by standing wave behavior (half wave patch antennas, half or full wave dipoles, slot antennas, small loops, etc). In wide band antennas, traveling wave behavior is dominant (various horn antennas, spiral antennas, flared/curved transmission lines, etc).
The difficult to understand antennas are the ones where multiple resonating structures are used to get wide bandwidth.
When you need a more specific answer, you should provide the community some specific information about what you want (frequency, radiation pattern, production method of antenna, polarization, bandwidth, study or actual design, etc).