SQL Server 2008 Reindexing/Rebuilding
I'd highly recommend using Ola Hallengren's index maintenance scripts, they're very widely used. 

https://ola.hallengren.com/sql-server-index-and-statistics-maintenance.html

I personally set the script to Reorganise at 50% and Rebuild at 80% (and only check indexes over 1000 pages). It's recommended to have this as an overnight run if you have the maintenance window.

Beware that the first time you run this it may have to do quite a lot of work.

As a side note, do you know that fragmentation is actually causing your performance slow down? It could be any number of things unless you've already narrowed this down. I'd recommend using Brent Ozar's scripts for a health check if you haven't already. Start with sp_Blitz and go from there.

https://www.brentozar.com/blitz/