Hi Yaser, first, check the literature, maybe you can find a guide RNA, which was already tested and worked for your particular gene.
After choosing your exon on NCBI or Ensembl, you can easily design the guide RNAs (it is recommended to order more than 1 gRNA, in case one of them won't work) on a vendors homepage or on other public site.
You can check our paper here, we knocked out a gene in rabbit, but the gRNA design is the same: