However, there are some basics.
Some folks pick up a Bible, flip open to a random text, and start expounding.
Some seek to address specific problems or weaknesses they perceive in their audience and pick texts which apply to the situation. A blessing of having a church calendar with its seasons and cycles is that variety is imposed on the pastor. Normally, the first reading is from the Old Testament, and has been selected to tie to the Gospel of the day. The second reading is usually from an Epistle, sometimes from Acts. This reading is integrated with the Gospel's theme during festival seasons.