courgettes

Glance back at some of the earlier questions on this site and you will see this is the most commonly asked question this year.
Sometimes the rot is simply because the flower wasn't adequately pollinated, due to lack of insects on wet or cool days. Sometimes the flower itself remaining on the fruit begins to rot and this spreads into the fruit.
If the fruit is rotting while still tiny, then try pollinating the female flowers with a little brush or feather, taking pollen from male flowers. If the rot is when they are larger, remove the flowers from the end of the fruit as soon as the fruit begins to grow away.