A rye starter in about a week
A rye starter is the simplest one to begin with. Stir equal parts rye flour and lukewarm water in a clean jar, cover it loosely, and keep it somewhere warm, around room temperature or a little above.
Stir it once a day. After the second day, discard most of it and feed what remains with fresh flour and water. The first days can smell odd, sometimes sharp or even unpleasant, and that is normal.
Somewhere between day five and day seven it should bubble reliably within a few hours of feeding and smell pleasantly sour. That is the moment it is ready to bake with. If it is slow, give it a few more days and keep it a little warmer.