I have a shakespeare sigma fishing reel that tangles up when I cast it, any solutions

The line needs to installed on the reel as it is on the new spool when purchased. Also check spool drag. If it is a spinning reel it is important to install the string correctly and use a quality brand of string.

Try attaching the swivel to a chain link fence and walk away feeding the line completely out. Then begin reeling in the line at a normal pace with the drag set at max, after 10-15 turns decrease the drag a full turn. continue doing this until the line is fully retrieved. then set the drag as you normally would before fishing. Sometimes the line gets twisted while fishing which cause the tangles and backlashes. Make sure you use a barrel swivel also.