Short answer: You can start with Blood of Elves which is the 1st book of Witcher Saga anyway.

Long answer: Long, long time ago Andrzej Sapkowski wrote a short story for a SF&F contest. He won the 3rd place and a lot of people loved it. So in years to come he wrote more short stories loosely connected by the main hero. They were subsequently collected and given rough timeline in The Last Wish. Then came the second collection of short stories The Sword of Destiny. By that time Sapkowski already knew he was going to write the novels and those stories are more relevant to the Saga's storyline (especially the titular one and the last one that sets up the events in Blood of Elves) but you cannot read them anyway because they haven't been translated and noone knows if they ever will be. So you don't really have a choice. You can start with Blood of Elves as stories in The Last Wish just share characters and world with it and were written just for fun (most of them are redone fairytales) and are not connected to the plot of the novels. Still the short stories are funny and well written so if you have time read them too.