If your site is in the Northern Hemisphere you need to aim your solar modules to the true south direction (the reverse is true for locations in the Southern Hemisphere) to maximize your daily energy output.
For many locations there is quite a difference between magnetic south and true south; please consult the declination map before you set up your mount structure. The solar modules should be tilted up from horizontal to get a better angle at the sun and help keep the modules clean by shedding rain or snow.
For best year round power output with the least amount of maintenance, you should set the solar array facing true south (or north if in the southern hemisphere) at an angle to the horizon equal to the degrees of latitude of your position. If you plan to adjust your solar array tilt angle seasonally, a good rule of thumb to go by is latitude minus 15° in the summer, latitude in the spring/fall and latitude plus 15° in the winter. Most mount structures provide for a seasonal adjustment of the tilt angle from horizontal to 65°.
Figure 9 - Sun Declination Map