Solar collectors and panels have to be correctly aligned to get the best performance; these angles can affect the amount of energy collected and are based on two factors: latitude and the season. Solar collectors will capture the most energy when most of the solar radiation falling on them is perpendicular to the collectors. You can see this from the image above.
Because axis tracking systems are prohibitively expensive and complex, we’ll ignore those, and assume the collectors are to be mounted in a fixed manner.
The general rule of thumb for fixed-mount solar collectors is to subtract 15° from the latitude of your location during summer and add 15° to your latitude during winter. Collectors should point towards the equator wherever possible.
In practice, it’s rather more complex. People want most heat during the winter and less during the summer. A fixed-size collector array that can easily produce their domestic hot water needs to be large enough to do so in the winter and angled appropriately for that season. During the summer that large array will not be best aligned, but it will be adequate to manage their hot water needs, given the longer hours of sunlight available.
As to choosing the location…the best is a roof facing the equator with a pitch that is close to your latitude + up to 15°, and that is not shaded by surrounding trees or buildings etc. Be aware that winter sun is much lower in the sky, so objects that appear to not shade or obstruct the collectors during the summer might well do so during the winter. If you are mounting collectors on a flat roof, make sure that they do not shade each other when the sun is low in the sky.
There’s a nice Android app called ‘Sun position and path’ which can help you determine these factors.
And there’s a good page on tilt angles here.