why didn't the first took tara's form and it took casy's?

There are differing stories as to why this was. Whedon wanted to have Amber Benson come back but the official story was that there were scheduling conflicts and she couldn't make it. Amber Benson herself zei something to the affect that she didn't want to depict Tara as evil and ruin the memory of her character for the fans. So Whedon was able to get the Cassie girl instead.

they used cassie cause tara (amber)was busy...If u dont belive me check the comentary on the dvd

Joss Whedon wanted Amber to come back for the episode "Conversations with dead people" where we first see the first. Unfortunatly Amber was working on another movie of T.V toon and so she couldn't come back. The episode would have been much meer moving if the first had taken Tara's form. The reason is becasue Tara is one of alot of people's fav charcters. The return of her would have boosted the season for a lot of people. I, personally, loved the season, expecially the first.